Complete Monster/Power Rangers

Though Power Rangers is a children's series, it isn't free of this trope and it features some villains who are particularly evil and despicable.

In Space

 * Darkonda from Power Rangers in Space, unlike many villains in The Zordon Era (first six seasons), had no qualities or loved ones to balance his evil. This sadistic and ruthless Bounty Hunter feigned loyalty so he could manipulate his way to becoming "King Darkonda.” Years before the series began, he was tasked to kidnap the young Karone and stop her dream of becoming a Power Ranger by having her memories altered and raised to be the villainous Astronema. When her older brother, Andros, learned it was Darkonda who kidnapped her, Darkonda proudly boasted that it was one of his finest accomplishments, frequently bragging about it to Andros for enjoyment. He was also involved in causing KO-35’s population to abandon their colony, using methods as the Barillian Bug infestation; he took the planet and its riches for himself. His crimes in the series proper include using the same bug creatures to infect the Rangers; forcibly fusing with his rival Ecliptor and use him a shield; and attacking the survivors of KO-35 with a monster that turns victims into coral whilst framing the Rangers for it. When Astronema learned the truth and Ecliptor refused to betray her, Darkonda mortally wounded him and had him turned into a cyborg, with implants that prevented him from rebelling. When they recapture Karone, Darkonda has cyber implants forcefully put on her, completely erasing any goodness in her. In the final episode, he makes his last bid for power by directly attacking Dark Specter with two torpedoes capable of destroying a planet. He fails and Dark Specter devours him while being destroyed by the last torpedo, crippling the evil forces. A power-hungry, untrustworthy, gleefully evil psychopath who endangers all around him whilst lacking a single redeeming quality, Darkonda is one of the vilest creatures in Power Rangers history.
 * The above-mentioned Dark Specter is the evil entity behind all other villains before who all fear and obey him, and is known as "Grand Monarch of Evil". Being evil incarnate and incredibly powerful, he can spread evil and brainwash people by Mind Control (he was the one who brainwashed Astronema/Karone and turned her into his personal right-hand henchwoman). His goal was to take over the entire universe, no matter how much destruction he had to leave in his wake in order to achieve this. Since he is a dark, demonic galactic overlord, is behind all the villains, and can spread darkness across galaxies, he is definitely one of the most evil villains in Power Rangers.

Lost Galaxy

 * From Power Rangers Lost Galaxy, we have Deviot. In a show full of villains with standards, Deviot had none. He was a manipulative, treacherous Starscream who would cheerfully throwaway his men's lives, thought brainwashing was a useful tool, and had no loyalty to anyone. Deviot destroyed Scorpius to get his hands on the cocoon that had been intended for Trakeena to awaken her true potential. When Trakeena, and not he, inherited control of the army, he sends two of his men to kill her; when she survives the assassination attempt, he kills the assassins to cover his tracks. He later brainwashed Loyax into fighting the Rangers, leading to his death as well. Then he hurtles the Terra Venture into another dimension, and finally fights Trakeena for the cocoon, until they both fall in. The absorption of his evil soul drives Trakeena completely over the edge, leading her to murder her mentor, Villimax, for refusing to fire on civilians, use her men as suicide bombers, and try to regain her human form by stealing the Life Energy of innocent people to power herself up. Totally untrustworthy, and out only for himself, Deviot proved that on his own, or influencing Trakeena, he could do more damage than any other villain in Lost Galaxy.

Lightspeed Rescue

 * There's also Queen Bansheera from Power Rangers Lightspeed Rescue; even in shows for adults, you'd be hard-pressed to find a worse boss. She has four main minions, and she's responsible for the deaths of all of them, draining the life from one, blowing another to smithereens simply for being in the way, and erasing the minds of the last two (one of said minions was her son!), forcing the Rangers to kill them. In addition, Bansheera attempts to open a gateway to the demon realm, allowing all demon-kind to pour out and overwhelm humanity until they're all wiped out completely. She's notable as the only Power Rangers villain to get a Fate Worse Than Death, as she ends up falling into the local Hell equivalent to be tortured for all eternity. What makes it fitting that Just shows that even when you try to make an imported version of Grandienne, you still get a monstrous bitch that is not at all kid-friendly despite being in a show for kids.

Wild Force

 * Master Org, the Big Bad of Power Rangers Wild Force: was unlike any villain before him or after, as he was a human who chose to be a monster. As Dr. Vicktor Adler, he was once friends with the parents of Cole Evans and even lusted after Cole's mother, hoping propose to her until learning Richard had already done so. Cole's conception and the scientists showing more interest in their family than in him only fueled his belief of being betrayed. After discovering the remains of the original Master Org, Adler swallowed them to gain his powers, using them to murder his former friends, Cole only surviving due to being hidden. Years later as the "resurrected" Master Org, he led the Orgs on a quest to destroy all of humanity, taking his frustrations caused by the Rangers out on the generals and dukes trapped in stone, all of them still very much alive and begging for freedom. When Jindrax and Toxica discover his deception, he brainwashes them into serving him. After they are freed and leave him, he decides to deal with the Rangers himself. He captures them and gleefully reveals to Cole he murdered his parents, which he "enjoyed every minute of." After being defeated and seemingly losing his powers, General Org Mandilok throws him off a cliff to his death. But he then gets revived merged with the true Master Org. Using his new servant Onikage, he had Toxica manipulated into getting herself killed and then murders Mandilok himself. He lies to Jindrax about being forgiven only to order Onikage to kill him. In the two-part finale, now in a new monstrous form, he destroys the Wild Zords (the zords in this season being alive with their own thoughts and emotions) along with Animus, and sends the Animarium falling into the Earth. He then covers the city of Turtle Cove in toxic rain, summoning an army of his Putrids to wipe out all humans, declaring Orgs as the rulers of the planet. Motivated by hatred and rejecting any offer of redemption given, Master Org showed than even in Power Rangers, humans were capable of great evil.
 * One thing that must be considered is how and why Adler chose to become a monster. Most other villains were either born that way, had a Freudian Excuse, were raised to be evil, from a species who were Exclusively Evil, or otherwise had no choice over what they were. Dr. Adler is so monstrous because of the simple fact that he has no real excuse at all for being a monster (along with other villains in this page) like nearly every other Power Rangers. He became the second Master Org solely to enact a petty vengeance on two perfectly innocent people who had no idea they'd even incited his anger in the first place.
 * Cole outright says only someone without a heart would do something like Adler did. Cole then checks and confirms Adler doesn't have one, having lost it to his self inflicted monstrosity long ago.

Dino Thunder

 * Mesogog from Power Rangers Dino Thunder is the evil reptilian alter ego of Anton Mercer who is determined to separate from Anton. Now, for his atrociousness: he lied to Trent (White Ranger) when he made the trade for the Dino Gems when he said he would give Anton (who he had separated from) back, then he tries to kill them. To power his machine, he takes the power from his right hand man Elsa when she points out he needed a power source to give it a boost to start it. He Mind Rapes his henchmen when they fall behind or fail or just to keep them in line, and he uses Mind Rape on his own minions more than on his enemies. And the fact that his master plan is to bring about a new Dino age. If you think about it, that would mean countless people would lose their lives.
 * What makes Mesogog even more terrifying is that he is completely emotionless. He does all those atrocities and feels absolutely nothing. And that voice: he didn't even raise it ONCE. And when he laughs, which is rarely, it is enough to make you shiver.

Mystic Force

 * The Master from Power Rangers Mystic Force, whose real name is Octomus, is a demonic entity intent on destroying everything good. Prior to the start of the series he brainwashed Leanbow into becoming his loyal servant Koragg. Once he emerges, he possesses a follower who had turned good and was trying to prevent his rebirth and mutates his body into his own twisted image; he then possesses Leanbow's son Nick directly, transforming him into a much more sadistic and vicious version of Koragg who laid waste to parts of the forest and Rootcore before he was saved. After being exorcised from Nick, he attacks and destroys the Mystic Mother's palace in a matter of minutes and kills both Leanbow and Daggeron when they investigate the destruction before dropping their corpses down before the Rangers. Finally he obliterates time itself, transporting the rangers to the future he plans to create where he's reduced the Earth to a barren, lifeless husk, for no apparent reason other than because he can.

Operation Overdrive

 * Power Rangers Operation Overdrive has Flurious. He was shown as being more ruthless than his brother Moltor and treated Norg (dumb, but innocent Yeti) badly. He was the first of the two brothers to openly declare hostilities, attacking Moltor for the Corona Aurora and declaring "it's every man for himself" afterwards. What really pushes him into this territory is what he does in the final episodes: he tricked his brother, who'd came on his hands and knees and offered peace, into handing over the Corona Aurora with an offer of friendship; however, Flurious kills him off in cold blood. And in finale, he freezes the entire city when he gets the rest of the jewels for the crown and decides to have Vella and Norg killed just because they weren't necessary to him.
 * Note,
 * Kamdor perfectly fits this trope as well. When his partnership with Miratrix started to falter, Kamdor did not appreciate her and offered no loyalty to her, and as such, started to ridicule and callously abuse her, to the point where she went mad and turn against him, and eventually using the Octavian Chalice to turn herself into an owl-like demon. However, after Miratrix got defeated, Kamdor told her she was just a pawn in his plans, and then trapped her in the jewel he was previously stuck in, and then presumably killing her afterwards. Kamdor's most evil act, however, was sending a meteor to earth to get rid of the Rangers once and for all, not caring if countless innocent people got killed in the process at all. He then brutally injured Will in a duel, but lost. A ruthless, traitorous, and truly sadistic villain motivated by greed, Kamdor was even more evil and certainly more competent than the main antagonist Flurious, his brother Moltor, or any of the other villains in the series.

RPM

 * Venjix from Power Rangers RPM in considered to be one of the scariest and most evil villains in the franchise. Destroying computer systems was part of his programming. But destroying 99% of humanity and turning the world into a barren wasteland was all his idea. We also find out his master plan: Remember all those attacks on Corinth foiled by the Rangers? All a decoy to distract attention from the real threat: the population of Corinth, most of which have been infected with the Venjix virus and have been slowly turning into hybrids over the course of the entire season so that Venjix can control them and conquer the city. And he had forced labor camps where experiments in cyborg creation were performed on the laborers. And when Venjix gains control of all the hybrids he experimented on and hid in Corinth, there are children among them, who don't even look a year over ten. And . Possessed of no good qualities, and with the highest body count in the history of the franchise, Venjix was truly one of the most evil villains in Power Rangers history.

Samurai

 * Power Rangers Samurai, despite being Lighter and Softer than RPM, has two Monsters:
 * Serrator. In "Broken Dreams", we find out he was the one who cursed Dayu and Deker, by tricking Dayu into becoming Nighlok: he said that he would restore Deker in exchange of Dayu becoming a Nighlok, and after she accepts, Serrator reveals that he made Deker half-human and half-Nighlok. Dayu realized that Serrator tricked her and he laughed at her misery. This is what made Dayu a miserable and tormented person. Mia, after seeing this, feels sorry for Dayu and Deker. Also, unlike usual Deal with the Devil examples, it is Serrator who came to Dayu and probably did that when he heard Dayu's call for help and wanted to ruin her life and screw her and Deker's lives, and usually, in deals, The Devil tells what the person has to sacrifice or what he will do to that person, but Serrator didn't tell Dayu about what would happen to Deker. And in his official debut episode, "Something Fishy" from Super Samurai, he destroyed an entire mooger army to show his power to Master Xandred. And he did all of this in only small amounts of episodes.
 * Master Xandred qualifies too. When he isn't drinking his medicine and yelling at people, he shows how evil he can be, being the Big Bad and all. He treats his followers horribly, threatening to kill Octoroo at one point, attempts to kill Deker because he was meddling in his affairs, and his plan revolves around killing Jayden. Also, unlike Octoroo, who sends Nighloks to cause misery, Master Xandred summons an army of moogers to cause meaningless chaos in the human world and destroy everything. In episodes "Broken Dreams" and "Test of the Leader", we see that his moogers destroy entire buildings, meaning that he is responsible for hundreds of deaths of innocent humans. He brutally punishes Arachnitor when he tried to usurp him by mutating him into a mindless slave. He also ruined Dayu's harmonium and attempted to erase her memory. She even says "I knew you were evil, but this..." It seems that Xandred's cruelty truly knows no bounds.

MegaForce and Super MegaForce

 * Vrak started off as The Dragon to Admiral Malkor in Season 1. After the deaths of both Malkor and Vekar, Vrak resurfaced in the season 2 two-parter "Vrak Is Back." Capturing both Orion and Robo Knight, Vrak slowly drained Orion of his energy and used it to brainwash Robo Knight. Vrak sent Robo Knight to kill the Rangers while he proceeded to drain Orion of the last of his power, intending to kill him and use his energy to power three giant drills that he could use to destroy Earth. In addition, when his subordinate Metal Alice helps Vrak and his near death, Vrak just leaves her to die, showing that he doesn’t care anything about her. He returned after his older brother, Prince Vekar, is defeated, cruelly mocking him for being too weak and actually thanking the Rangers for killing him so that he could claim the throne for himself. Having kidnapped Robo Knight and Orion, he drained the life force of the latter, forcibly reprogramming the former to attack his former friends while constructing drills that burrow into the Earth's core. He tricks the Rangers into destroying his two remaining monsters, who become new drills to complete his plan for hostile takeover of the planet quicker. Lacking empathy for both allies and enemies, Vrak is one of the franchise's cruelest aliens to invade the Earth.

Dino Charge and Dino Supercharge

 * In spite of this show being one of the more campy ones, there are two types of monsters that are misplaced in this show, yet still managed to be there:
 * Lord Arcanon certainly counts as one. A psychotic warlord with an unquestionably mad addiction to power, he is the one who drove the events of Dino Charge by hiring the mercenary Sledge to find the Energems and bring them over to him. However, the bounty hunter's own greed and hunger for power made it so that Arcanon could never possess the Gems. Angered, he sends his traitorous minion Singe over to find them. He is also the one responsible for creating the split personalities of Heckyl and Snide, an act which would soon lead to his downfall. Upon meeting the Rangers, he wastes no time on giving them a difficult time, using his minions as batteries for his new Megazord and armies, and then forcing Snide to work for him. As another kick to the teeth, he pulls out a dark Gem from his book, and tortured the Rangers with his new powers, all while laughing maniacally at their suffering. Ironically, it took Snide's betrayal and an unexpected reunion with Sledge that led to his death. Sociopathic, megalomaniacal, and completely insane (despite his appearance), Arcanon darkened the show for every scene he appeared in.
 * The aforementioned Singe also counts by a long shot. A sociopathic, traitorous, narcissistic, and sadistic master manipulator, Singe is a devilish candle-themed alien who seemingly wants nothing more than to please his equally cruel master. He infiltrates Heckyl's hideout, and arranges his meeting with the Rangers, in which he sarcastically claims that he has a headache, and that the only cure is to kill the Rangers. After being defeated multiple times, Singe was caught as a traitor and escaped, much to Heckyl's anger. As if treason wasn't bad enough with this guy, he then helps his master torture the Rangers, and possibly kill millions of innocent lives, all in search of the Gems. He also helps in abusing Heckyl and Snide, in their separate order, which consequently led to to his master's deaths. (It is highly possible that Singe would betray his own master if the opportunity arose, but he never got the chance to explain his own motives.) The Deviot (yes, a reference to that other villain on this list) of the Dino Charge era, Singe has done more damage perhaps even more so than his master.

Power Rangers (2017)

 * Rita Repulsa began her career as the Green Ranger before her ambition drove her to slaughter her comrades and mortally wound her former friend Zordon. Resurfacing countless years later, Rita descends on the fishing town of Angel Grove and starts brutally murdering innocent people for their gold to regain her powers — in one instance tearing a homeless man apart to get at his gold teeth — and burning down an entire populated building to demonstrate the power of her Putties. Baiting and then killing a man to draw in the Rangers, Rita tortures the location out of Earth's Zeo Crystal out of the Rangers and then callously drowns Billy in front of the others once he wears out his use to her, finally rising up a monstrous Zord of her own making that she calls Goldar to decimate Angel Grove and take the powers of the Zeo Crystal. Rita's ultimate goal is nothing short of absolute godhood by harnessing the power of the Zeo Crystal – consequentially killing off all life on the planet, something Rita is only too gleeful to do. Rita shockingly contrasts the campy nature of her original namesake and is ultimately nothing less than a psychotic, murderous monster who barely resembles her original counterpart.

Comic Books

 * Mighty Morphin' Power Rangers (BOOM! Studios): Lord Drakkon is an Evil Counterpart of Tommy Oliver from an alternate timeline where he refused the Rangers' offer to become their Sixth Ranger, and willingly became a servant of Rita Repulsa, and aided her in her conquest of Earth, and ultimately a ruthless tyrant who rules his dimension with an iron fist. He kills Jason and took his white Ranger powers and transformed into a Green/White Ranger hybrid, brainwashed his dimension's Kimberly into the Ranger Slayer. During the Shattered Grid arc, after escaping captivity, Drakkon regains his powers, and outright killed the original Tommy. Drakkon resumes his reign of terror by invading every Power Rangers' multiverses to take their powers for his own purpose. His body counts also includes his Zeo and Dino Thunder counterparts, Ninjor, Zordon,  Alpha 5 and 6, and ultimately Finster 5. Drakkon's ultimate goal is to recreate the multiverse in his own liking, going so far to get into the Morphin Grid, defeating the Emissaries Three and using the Heart of the Master. Defined by his God complex and inability to understand the bonds of every Power Ranger known to the multiverse, Lord Drakkon is everything what the Tommy we all know and love is not, and is easily one of the darkest villains in the franchise, even by Ranger standards.