Complete Monster/Fullmetal Alchemist

Fullmetal Alchemist has many evil villains, but a few stand out as particularly horrible.

Manga and Brotherhood
The manga and second anime series, Brotherhood, has some surefire examples:
 * Shou Tucker, a Mad Scientist who transforms his own four year old daughter and the family dog into a Half-Human Hybrid, partly to see if he can and partly to retain his State Alchemist license. He had previously done the same thing to his wife. Making Tucker even more abhorrent is his complete refusal to own up to what he did: he acts as though the Elric brothers shouldn't have a problem with his actions, assumes that anybody else put in his desperate situation would have done the same thing, and spends the rest of his time alive whining about how "nobody understands" that he did it For Science!, though in truth it was because he was either too lazy, or too devoid of talent to come up with another way of keeping his status. He's the only character who even comes close to triggering Al's Berserk Button, and what he did leaves a stain on the Elric brothers' memories for the rest of their lives.
 * To put things in perspective: of all the characters who died in the story (shortly after Tucker's arrest, Anti-Hero Scar breaks into his house and bloodily murders him), he's the only one who was sent to Hell in the "In Memoriam" omakes. The authoress Hiromu Arakawa allowed even characters like Envy or Kimbley to get to Heaven, because she thought there was still something worth saving about them. With Tucker, she was absolutely merciless, and with good reason.
 * Even after he's died, it gets worse. Later on, it turns out that what he did was totally and completely pointless since the military already had much better chimeras. Imagine straight up humans who can harness the instincts of a specific animal and even turn into that animal temporarily in order to fight better, like the Lizard with none of the Super-Powered Evil Side effects. Compare that to the paralyzed husks of pain that became the Tucker family.
 * The Big Bad, Father, is a being from beyond the Gate given form in a small flask and named Homunculus by the nameless slave who would become Van Hohenheim in the Xerxes empire. Desiring to escape the flash, Homunculus would trick the king of Xerxes into a ritual that slaughtered multiple people to initiate an alchemy ritual that killed everyone in Xerxes and split their souls between Hohenheim and Homunculus, to the former's horror. Later founding the nation of Amestris and creating the Homunculi, he took the name of 'Father', guiding Amestris to become a military dictatorship that initiated military conflicts to provide bloodshed for an even greater ritual, including the genocide of the Ishvalan people. Intending to kill and steal the souls of tens of millions so he could devour God, or Truth itself, Father demonstrates cold sadism when he faces Hohenheim and his children again, mentally torturing them as he also abandons or tries to absorb his own Homunculi children. While he sees himself as an existence above humans, this is simply a reflection of Father's arrogance and pride, desiring to ascend ever higher, no matter who or what he has to destroy to gain ultimate knowledge and power.
 * The unnamed and utterly heinous Dr. Goldtooth is essentially Tucker 2.0. A Mad Scientist for the State, he crosses the Moral Event Horizon in one of Bradley's flashbacks, when he's shown cackling with glee as he puts one subject after another through the process intended to create Wrath — a process which horrifically kills the first twelve candidates, deconstructing their bodies while they are still alive. In the present day he only gets a few scenes, but they are more than enough to show us how horrid he is. He uses his medical alchemy to heal Mad Bomber Zolf J. Kimbley so that he might be unleashed upon the heroes again. He later looses the remaining Fuhrer candidates (all of whom have been reduced to an essentially mindless state) on Ed, Roy, Hawkeye, and Scar to prevent them from interfering with Father's plans to destroy the country, activates the Transmutation Circle that sends Ed, Al, and Izumi into Father's presence, and finally, has one of the candidates slit Riza Hawkeye's throat in an attempt at forcing Roy to perform human transmutation and open the Gate. Perhaps most importantly, he's privy to Father's endgame (unlike Central Command, who believe they are all going to receive immortality) and simply doesn't give a damn. No explanation for his actions is ever given, and he spends all of his appearances sporting a Slasher Smile and bragging about his achievements as a scientist.
 * From the Brotherhood movie, we have Security Chief Atlas, who killed the movie's female protagonist's parents, then stole her brother Ashley's skin, and used it as a disguise in order to trick the rebel group, Bats, into making a Transmutation circle for him in order to gain power to rival other nations. He also stabbed the guy who was secretly in on this as well because he needed more sacrificial blood. He gets his comeuppance when a Julia and  kill him in the end.

2003 Anime
The 2003 anime version has its own set of monsters:
 * Mugear wanted to introduce a toxic chemical into pregnant women to produce an alchemical amplifier. Before that, he had no problems producing said amplifier at the cost of the villagers' health. He also murdered Nash Tringham, the inventor of the method, after he refused to use the method or continue the research on it due to the health risks for the people of his village and tried to sabotage its use by Mugear.
 * Brigadier General Basque Grand (a Reasonable Authority Figure in the manga) is portrayed as a sadistic Colonel Kilgore and one of the key players in the Ishbalan massacre, who laughed maniacally as he gunned scores of civilians down with his Philosopher's Stone, authorizing its use in order to turn the massacre into a massacre. Grand forced Roy Mustang to kill Winry's parents because they were treating Ishabalan patients, even though they were also treating Amestrian soldiers. When he discovers what Shou Tucker had done to his daughter, he decides to cart her away for study, presumably so he could bully Tucker into making more chimeras lest he lose his State Alchemist license. When Scar blows his brains out, it is very, very satisfying.
 * The Big Bad, Dante, is an obvious one. She manipulates and abuses her homunculi servants and sends them to their deaths if they don't do exactly what she wants. Their desire to become human leads them to question if she really intends to make them human (she doesn't), and she can't have that. She plots the destruction of an entire nation in the hopes that it will convince someone to create a Philosopher's Stone, which she can then steal in order to live a few years longer (when she's already Really Seven Hundred Years Old). And that's on top of already being a body-snatching bitch who plans to hijack the body of Ed's girlfriend Rose, who already went through serious Break the Cutie, including, solely so that she can force him to have sex with her. Ed is both a teenager, and her ex-boyfriend's son. She later shatters the mind of one of her Homunculi servants when he dares to miss his dead friend, turning him into a mindless eating machine.
 * Pride is the military dictator of Amestris and one of Dante's most trusted homunculi. He is the mastermind behind the events that caused the genocide in Ishbal, and worse yet, repeats the same actions in Reole in order to lead his own nation in destruction. At one point, he even releases infamous Mad Bomber Solf J. Kimblee from prison and unleashes him upon Reole citizens in order to take care of the deed as fast as possible. While mainly assisting Dante in her plans to create the Philosopher's Stone all over again, it is made clear that Bradley enjoys committing these deeds for the sake of "purifying" the population and expanding the country's territory. After Martel finds out about Bradley's true identity, he proceeds to murder her cold-bloodedly in front of Al in order to keep her silent. It is not until the final arc of the series when Bradley reaches the level of an irredeemable monster, when he plays a sadistic cat-and-mouse game with Roy Mustang, torturing him both physically and emotionally, even bringing up the tragic fact that he couldn't save his best friend from a certain death. When his own son steps in the hopes of helping his father, Bradley breaks his neck while ranting about being an agent of God.
 * Envy is a malevolent sadist who uses his Shapeshifter Guilt Trip by taking on the forms of people's loved ones before killing them. He is the one responsible for murdering Maes Hughes, by impersonating his wife and rubbing it at his face before killing him. Envy is mostly seen manipulating Amestris's government behind the scenes with a string of mind games and murders, while Pride controls the military as King Bradley. This all culminates in him killing Ed when he finds out that Envy is in fact, his half-sibling. Unlike other Homunculi, who wanted to become humans, Envy was the sole exception, only working for Dante so he could kill as many people as possible, abusing both other Homunculi and prisoners in the process simply for laughs. His Parental Abandonment as a Freudian Excuse doesn't even come close to justifying his homicidal mania and Envy completely undermines it in Conqueror of Shamballa when he decides to murder his father rather than trying to reconnect with him.
 * Zolf J. Kimbley: Unlike his manga counterpart, who has a definite case of Moral Sociopathy going on and believes Rousseau Was Right, this version is a Psycho for Hire, Nietzsche Wannabe, and Misanthrope Supreme par excellence, who Loves the Sound of Screaming and isn't afraid to share it with you. He committed genocide in Ishbal, tortured Scar by blasting him apart one piece at a time, blew up his fellow prisoners in order to escape from prison, betrayed Greed to Frank Archer, tries to murder Al soon after his reinstatement, reenacts the Ishbal genocide in Liore, and finally dies while trying to take Al and Scar with him, succeeding in killing Scar. A truly appalling mix of rage, hatred, and pure nihilism.
 * Barry The Chopper: Unlike his manga incarnation, who was Played for Laughs, his iteration in the 2003 anime was far more vicious and horrifying. Barry was once a simple butcher living in Central City who had grown bored with his life of cutting up pigs and cows. One day, to see what it felt like, Barry decided to chop up his wife and quickly found that he loved the sensation. What followed was a string of women being brutally murdered. Barry's killing spree came to a temporary halt when he was arrested after trying to kill the Elrics' childhood friend, Winry. Rather than be executed, Barry was experimented on, had his soul bound to a suit of armor, and became one of the guards for the Fifth Laboratory. After the lab's destruction, Barry slaked his bloodlust by joining a group of mercenaries to slaughter a camp of harmless Ishvalans. An utter sociopath, Barry killed people for no other reason than because he enjoyed it. When asked by Alphonse if Barry was afraid that he wasn't the real Barry and was just an artificially constructed soul, like Barry accused Alphonse of being, Barry stated that it didn't matter if he was the original Barry or not, for so long as he had the ability to kill people, he loved who he was: "I kill therefore I am."
 * The Conqueror of Shamballa: Thule Society Chairman Dietlinde Eckhart is the Big Bad and an occult supporter of the Nazi party trying to reach the titular Shamballa to gain the power to conquer the world. Capturing Envy to open the gate to the other world, Eckhart keeps the homunculus docile by allowing him to bite into Hohenheim, leaving the man in agony. When Hohenheim sacrifices himself to open the gate, Eckhart, no longer needing Ed's alchemy, shoots him and leads an assault into his world. Enthralled with her newfound ability to use alchemy but fearing others with the same power, Eckhart tries to destroy the very world she once sought to conquer, a xenophobic hypocrite who would not tolerate anyone but herself having authority.

2018 Live Action Film

 * Shou Tucker, the Sewing-Life Alchemist, is arguably even worse than his anime counterpart. Revealed to have fused his own wife with a dog to turn her into a chimera, a twisted parody of life that suffers in perpetual pain. Later performing the same treatment with his own daughter and her dog, Tucker defends his actions by claiming it is completely natural to perform unethical experiments solely on the basis of being able to, even having the gall to claim that Edward Elric is no different from him. He'd also planted seeds of doubt in the mind of Ed's younger brother Alphonse to make him believe his memories may be false and he might not truly be Ed's brother, for no reason other than to assert some form of control over Al, whom he saw as a special test subject for him. After being beaten and imprisoned by Ed, Tucker is later broken free and joins a military coup where he helps to create countless Mannequin soldiers to unleash upon the civilian populace solely for spite against Ed without a single care for the thousands of lives that will be lost, also threatening to blow a hole through Winry Rockbell's brain and torture Alphonse if he didn't lay his mechanical arm down on the transmutation circle at Camp 5, and gleefully enjoys breaking Ed's spirit by showing him the real Philosopher's Stone in his possession and revealing it's origins involving hundreds of living human souls having been sacrificed in death camp experiments before getting ready to shoot Ed in cold blood while he's defenseless to stop him.
 * General Hakuro is a ruthless military officer seeking a military coup. Manipulating conflict, death and murder across Amestris, Hakuro recruits Tucker and reveals the truth of the Philosophers' stones as the amalgam of fused human souls and lives, flashbacks showing that Hakuro had thousands of prisoners and innocents painfully melted down to create thousands and thousands of stones. Upon confronting Edward, Hakuro plans to unleash the cannibalistic Mannequin Soldiers on Amestris to kill until Hakuro can seize control of the country, driven by nothing but a hunger for power to the point of even declaring himself the future ruler of the entire world.