Complete Monster/Ben 10

Ben 10 and the other Cartoon Network series' set in it's universe feature more than a few of these:

From Ben 10

 * Vilgax is the Overarching Villain of the franchise, and Ben's most personal enemy by far. He's a psychopathic galactic warlord who, after banishment from his homeworld due to his evil, went on to assault and conquer numerous planets across the cosmos, outright annihilating the entire planet of Petropia and its billions of inhabitants to make an example of those who resist him. After discovering the existence of the Omnitrix, Vilgax becomes obsessed with possessing the gadget to modify it for his own gain, and spends much of the first series sending wave after wave of his forces to Earth solely to kill Ben Tennyson, a 10 year old boy, and steal the watch from him, later trying to viciously saw the boy's arm off and blow up the entire Earth for the trouble it has caused him. In Ultimate Alien, Vilgax becomes the herald for Diagon before betraying and killing his master and stealing his power, hoping to use it to mind control all life in the universe. In the final series, Omniverse, Vilgax, though at first seeming to have decayed in his evil, pulls out perhaps the most wicked scheme in the entire franchise, using a Chronosapien Time Bomb to wipe out all dimensions and timelines, his own included, to leave only a single, more submissive universe for him to conquer and rule. Vilgax was a madman who would kill first billions, then insurmountable trillions, just to satisfy his raging desire to be the most powerful being in the universe, and stands out as the most iconic villain in Ben 10 for this very reason.
 * While the original series' incarnation of Kevin 11 was an eleven year old sociopath of a mutant who'd destroy 2 trains of innocents for cash, killed a police officer with no regrets, killed an alien just to take a ship and crew he'd use to spread terror and death over the galaxy,turned turns down 2 chances at redemption (with Ben even calling him out on how his Freudian Excuse can no longer be a valid way of excusing away his crimes), and when he and Ben are forced to work together, he ends up trying to kill Ben through impaling him and whose attacks on Ben and his family were more vicious than most foes, save for the other examples on this page, redemption isn't impossible for the kid (and in fact, we do seem him redeemed in the canon's sequel series). However the Alternate Timeline episode Ken 10 shows us what he'd become if he'd stayed on the road to evil: enter Kevin 11000, an adult version of Kevin 11. Unlike his younger self, who merely straddled the line of the trope prior to rehabilitation and redemption, Kevin 11000 is a fully fledged case as he now seems much more in control of his emotions as he is in control of his powers and is fully aware of what he's doing - he just continues to do terrible things simply because he gets enjoyment out of it. To spite his old nemesis, he tries to murder Ben's 10 years old son and would've likely killed his own son just to do it - that's how little he cares for anyone who's not himself.
 * The Ben 10: Ultimate Alien series gave us Aggregor, who kidnaps aliens from different planets to zap their powers and use them as his own. What cements him as a Complete Monster is when he kills a peaceful turtle alien, all on SCREEN, and then turns its powers against its brethren before kidnapping the main one. He then kills the plumber from Season 2 of AlienForce by stabbing him in the back when he found the prisoners. He manages to top himself, when learning of the Omnimatrix, he tries to force Ben into a form he wanted after a no-holds-barred shock down by stabbing him right in the Omnimatrix icon with his spear and twisting it. If not for the Heroic Sacrifice of the jellyfish alien, Ben would have died, since both Kevin and Gwen were down. His vilest crimes were when he nearly destroyed an inhabited world by stealing a component from the device that kept the planet from breaking apart, and when he attempted to kill and absorb a baby alien capable of Reality Warping to complete his ascension into godhood
 * Diagon, who's an Expy of the original series' Vilgax in characterization, only with the Cthulhumanoid played up to the ultimate extent. He's also the one who a temporarily corrupted Charmcaster offers 600,000 living souls to in exchange for a wish, and possibly even had a hand in Charmcaster and her uncle Hex being corrupted to start with!
 * Malware, the true Big Bad of Season 1 of Ben 10: Omniverse and Khyber's boss, is a psychotic Galvanic Mechamorph born with a malfunction forcing him to sustain himself by cannibalizing people of his own species in exchange of the ability to copy technology by consuming it. His onscreen actions include trying to crash a school bus just to distract Ben, . He especially stands out as this trope amongst Ben 10 villains, because, despite being in the most goofy and light-hearted entry in the franchise, his viciousness and cruelty rivals Ghostfreak and Aggregor.
 * The final Big Bad of Omniverse, Maltruant, is a rogue Chronosapien bent on ruling all of time and existence who masterminded the original Time War in his first bid for universal control. He was defeated and his body separated across time by Professor Paradox. Seeking to restart the Time War, he hired Dr. Psychobos to help repair himself. After his Co-Dragons Subdora and Exo-Skull complete repairing his body, Maltruant turned on Dr. Psychobos, easily defeated Ben Prime and Ben 23, and left both for dead in Mad Ben's dimension with his minions to restart the Time War. Once he travels to the time period of Ben 10,000 to steal the Anihilaarg and the Dwarf Star so he can create a more deadly weapon, Maltruant traveled back to the very beginning of the universe and snuck aboard the Contemelia's ship, where he switched their Anihilaarg with his own in order to recreate the universe in his image. Despite Laughably Evil traits, Maltruant cemented himself as one of Ben's most monstrous enemies through his power lust and egomania.

From Generator Rex

 * Van Kleiss. Held New York City after he had his men get nanite rich soil from his realm underneath it, all the while abusing his Diplomatic Immunity so Rex and Six couldn't touch him. Drinks the blood of Evos who outlived their usefulness or failed him and views everyone and everything as pawns. He even kills a few cops on screen and nearly kills Rex, Six, and their Five-Man Band. Van Kleiss makes a habit of petrifying E.V.O.s by sucking the nanites out of them, or turning regular people into E.V.O.s to act as muscle for him, not caring that such an action could leave them permanently transformed. In his bid for power, he murdered Rex's parents, experimented on his Woobie, Destroyer of Worlds follower to amplify her powers while causing her excruciating pain, and threatened a little girl to force her father to turn the people of New York into his zombie army. Is also implied to be behind the entire disaster. Karma caught up to him, though. Twice. First he "dies" when he tries taking some super advanced nanites. Second, when he comes back, Rex pulls off a masterful gambit - have him rein the out of control nanites into line, then claim he's doing a Face Heel Turn...then he cures Kleiss, rendering him completely human and powerless. Unfortunately, his brilliant and wicked mind was still his greatest asset, and he used it to reclaim power and commit further acts of terrorism afterwards. Van Kleiss eventually attempted to absorb all Meta Nanites in order to gain power to destroy and dominate the Earth, but this backfired when he was again cured, for good this time.
 * Branden Moses, who announced a breakthrough in nanite research that allowed him to dramatically reverse E.V.O. mutations and gave a faux demonstration using an E.V.O. and either an actress with a set of prosthetic tentacles or an E.V.O. with tentacles, playing the part of the "cured E.V.O." Having convinced hundreds, possibly thousands, of people (including Dr. Holiday, who added her sister Beverly Holiday to Brandon's ranks) that he had made a breakthrough, dozens of intelligent but physically deformed, incurable E.V.O.s lined up to be cured. Instead, they were kidnapped and brought to a lab in the Arctic circle where their mutations were amplified and were ready to be auctioned off to the warlord with the deepest pockets.
 * Hunter Cain is a fantastic racist who kills E.V.Os left and right even when Rex is trying to cure them. His racism stems from bitterness and rage over the death of his wife, which he blames E.V.Os for, but rather than hunting them to avenge his love, his focus is purely on satisfying his hatread instead. In his first appearance, he used his goon squad to piss Rex off in an attempt in discrediting him, and finally, he cloned E.V.Os to lure Rex out, was prepared to sacrifice his own men, and then tried to murder Rex when he was unarmed. In his next appearance, he got hired to drive a family of werewolf-E.V.O.s out of a Mexican village, but actually planned to kill the entire population, not caring if any of them were children. He took delight at a chance at revenge on Rex, saving one anti-Nanite bullet for him. He was even willing to kill their non-E.V.O. caretaker for defending them. In his final appearance, he works alongside other enemies of Rex to get revenge, not caring that one of them, Noface, is an E.V.O. Rex even calls him out on it. He turns on them in an attempt to kill Rex himself. Violent and hypocritical, Hunter Cain may act like a hero of the people but is really a sociopath who will use any excuse and break even his own code to hurt anyone he hates.
 * Quarry. He appears 3 times. The first as a gang boss, manipulating several children, and killing underlings who fail him. The next time, he gets a device to destroy organic matter, and decides to blow things up. Later, after a run in with the Big Bad puts him in bad shape, he reappears, using Mind Control devices from Providence to make any runaway work for him. He plans to create an army of controlled Evos, then he tries to control Rex, and finally crush him after that fails. To top it off, he's voiced by Mark Hamill.

From The Secret Saturdays

 * V.V. Argost, big time. As the Himalayan Yeti, Argost murdered people for pleasure, preying upon the humans who wandered through the mountains and taking "souvenirs" off their bodies. Two of his victims were Drew and Doyle Saturdays' parents. When confronted with this, Argost only says he can't be bothered to remember all of the people he's killed. Throughout the series, Argost was obsessed with finding Kur, a god who had the power to control all cryptids. Eventually Argost discovered that Zak Saturday, was Kur himself, and so Argost began plotting to steal the boy's power. Eventually Argost managed to succeed in this by draining the god power out of Zak Monday, the evil Alternate Reality counterpart of Zak Saturday. This had the side effect of killing the boy. With his newfound powers, Argost ordered his cryptid army to wage war against every major human capital city, with the ultimate goal to Kill All Humans and rule all cryptids. Though already possessing a god's power, Argost craved more power still, and so he killed Zak Saturday to drain his power as well, just as he did to Monday. Ever the showman, Argost took to committing atrocities with a theatrical flourish and dry humor, relishing the screams of his victims and considering the destruction of mankind as just another one of his grand performances.