"Understand this. So long as you stand in my way- so long as anybody stands in my way, I will respond by killing. Murder on an industrial scale. Because in the final analysis, I would happily wade across a river of corpses, chest-deep in grease and engine oil just to crush the spark of the last Autobot standing. And I would do so not simply as a means to an end, no. I'd do it, Prime, because it would give me pleasure."
Megatron is a self-proclaimed tyrant and would-be Galactic Conqueror, who named himself after his world's equivalent of The Antichrist. A user and abuser of his followers, Megatron divides the world into enemies and pawns, killing or enslaving the former, and ruthlessly exploiting the latter. A gloating sadist, Megatron never kills anyone quickly when prolonged torture will do, considers his own allies expendable, and hopes to bring Cybertron, and eventually the galaxy, under his heel. His more notable crimes include: extracting half of Rampage's spark, placing it in a vise, and using it to torture Rampage into submission; ordering Dinobot to kill Quickstrike, one of Megatron's more loyal soldiers (though this is a relative term), after Quickstrike lost to Dinobot in a sort of re-entrance exam into the Predacons (Dinobot refused); an aborted genocide of humanity's common ancestor; attempting to rewrite all of his history by killing the original Optimus Prime; and finally, sacrificing all his remaining troops in order to gain control of the Nemesis and set out to overrun the galaxy. His defeat doesn't stop him either - when he returns in Beast Machines he's gotten worse, picking up a serious god complex along the way. Seizing control of Cybertron, Megatron extracts the sparks from everyone on the planet, reducing them to mindless drones. Accquiring a hatred of all organic life, Megatron tries to scrub Cybertron clean of it, resulting in the near destruction of all life, robotic and organic, on his home planet when his last mad grasp for power finally fails.
Tarantulas is Megatron's Evil Genius, and a schemer and backstabber second to none. Planted as The Mole in Megatron's operation by the Predacon Secret Police, Tarantulas is a cackling Mad Scientist, who enjoys performing gruesome experiments on his victims, and eating anything or anyone he catches in his giant webs - it doesn't matter whether it's organic or robotic, he admits to enjoying the act itself more than the meal. He brainwashes several Maximal protoforms into becoming Predacons, most notably Blackarachnia and Inferno, and eventually reveals that he hopes to usurp control of the team from Megatron. That's all in Season 1. Tarantulas only gets worse after this. He exploits and Mind Rapes Blackarachnia, leaving her with a paranoid terror of him, and later swears a vendetta against her, promising to hunt the other spider down and kill her for daring to defy him (if this sounds suspiciously like Domestic Abuse, it should). He tries to destroy The Ark, which would have lead to Unicrondevouring Cybertron in the future, and the extinction of the Transformers as a species. Ultimately betraying both sides, Tarantulas is revealed as the spawn of Unicron, and attempts to use alien technology to wipe out Autobot, Decepticon, Predacon, and Maximal alike, swearing that when he's done with his own species, he will return and commit genocide against the aliens themselves. Void of loyalty or conscience, Tarantulas makes even his own nominal allies uneasy, a fact that he is downright proud of.
Lockdown killed his old master (as well as The Obi-Wan to Prowl and Jazz) to steal Transformer infants protoforms in order to sell them and tortures his bounty hunting victims by cutting off their specific upgrades and using them to make himself more powerful. He enjoys hunting more for the thrill of the chase and the chance to vivisect his victims than anything else.
Another true monster is the human supervillain Meltdown. After being mutated by his own experiments, he tries to kill his rival, Isaac Sumdac, in a fit of rage whilst blaming him for all his problems (including his self-inflicted transformation). Later, he becomes so obsessed with creating a human transformer, he not only mutates at least two innocent humans into gruesome hybrids mish mashed with animal parts, he decides to kidnap Sumdac's innocent eight-year-old daughter as his newest test subject (deciding to experiment on children in general now). When Blackarachnia breaks him out of jail in exchange for him eliminating her organic half, he double-crosses her and tries to destroy her transformer half - a deed which, if not destroying her soul completely, would have left her with a completely organic body she's disgusted with. But damn, can he dance. He also forces the Dinobots to work for him by painfully burning them with his acidic touch until they follow his commands. He hurt them so badly, that they were willing to turn on Bulkhead and Prowl, their only friends among the main cast, just to stay in Meltdown's good graces. No wonder Meltdown is the only human villain in Animated played as a deadly serious threat to the Transformers. Dancing aside.
Silas is one of the few human villains in Transformers history to hold this trope. In "Operation: Breakdown", he captures Breakdown, tortures him, tries to have him vivisected, and removes one of his eyes. In Crisscross he allies M.E.C.H. with Airachnid, a Complete Monster herself, to get his hands on another Cybertronian (this time, Arcee) and was perfectly willing to let Airachnid kill Jack Darby, a sixteen-year old teenager, and his mother to do it. He even states a few human lives are worth it for Arcee's tech. In "The Human Factor", after his men upload him into the now-dead Breakdown's empty shell, saving his life in the process, he thanks them for that and their years of loyal service by thanking them personally and then slaughtering them. He then betrays the human race to Megatron, seeing himself as a superior being, razing a military base just to get Megatron's attention, then attempting to blow up everybody inside Raf's house (for reference, that's the human trio, Agent Fowler and Raf's mother). It says something that Knock Out is completely disgusted by him, and Megatron calls him an abomination, though admittedly Megatron hates all humans.
Airachnid is by far one of the evilest villains of the current cast. She's a self-styled "ex-Decepticon" who spent her time in the Great War capturing, torturing, and slowly killing any Autobots she could find. For certain victims, like Arcee, she'd go out of her way to find people they cared for and murder them before their eyes as part of her fun. After the war left Cybertron, she struck out solo to pursue a new hobby: hunting endangered (and soon to be extinct) species for sport and collecting grisly "trophies" of her kills. Should she find an attractive species that isn't endangered, not to worry, she'll ensure that they are by the time she's done. After coming to Earth, she's decided to make humans her new toys, as Jack found out the hard way. Airachnid's not under orders or attempting any form of grand scheme, she just finds it fun. There's also the utterly brutal way she kills Breakdown, tearing him into pieces. She's an unholy blend of Tarantulas, Beast Wars Rampage, and Lockdown pressed into a spider's shape and let loose on the universe.
Megatron/Galvatron in this continuity - though a Noble Demon in "Armada" - eventually enters Complete Monster territory at about the time he uses the Super Energon in "Energon". In "Cybertron", he goes into full Omnicidal Maniac mode. The point when he's absolutely solidified to be one is during the confrontation between him and the planet leaders in the black hole. When he reveals his A God I Am plan to destroy the universe then recreate it in his own image, he's asked how he plans to save his followers from the fate of the rest of the universe. He doesn't. As far as he's concerned, if they can't survive the end of the world, it's not his problem. At this point, every character present views him as this trope.
"It's over Galvatron. You don't care about anybody, and nobody cares about you."
Sideways, Unicron's dimension-traveling Herald became one of these during his Armada incarnation. Posing as a drifter with no particular allegiances, Sideways befriends Hot Shot and joins up with the Autobot cause, while plotting with the Decepticons behind their backs. Pretending to be Megatron's hostage, Sideways manipulated Hot Shot into handing over the Star Sabre to Megatron, and then revealed his alliance with the Decepticons, laughing about how Hot Shot was a fool to trust him. Now one of Megatron's most trusted men, Sideways planted seeds of mistrust in the Decepticon ranks, tricking Starscream into turning on Megatron, while secretly communicating with Unicron about their weaknesses. Found out, Sideways fled, only to return using Nemesis Prime to attack the Autobots and Decepticons alike, and persuading Decepticon Thrust to ally with Unicron, only to discard him the moment he is no longer necessary. Laughing Megatron and Optimus Prime's faces, Sideways reveals that he stoked the flames of Autobot/Decepticon conflict so that he could use their mutual hatred to reawaken and empower the sleeping Unicron. The perfect con artist, Sideways takes his moral cues from his master, and is loyal only to his own dream of multiversal destruction.
The Marvel UK version of Galvatron. In his first appearance, he dares four Autobots to shoot him, shows their weapons aren't enough to stop him, then beats them all senseless anyway. Then lobotomizes Jazz and gets him to beat them senseless. By the "Wanted: Galvatron" arc, he's planning to destroy half the coast just to gain a new power source. By "Time Wars", he seems to have declared war on the entire Transformers race, being practically the only opponent in a battle that leaves as many as a dozen dead (it's hard to get an accurate body count).
Overlord in the Last Stand of The Wreckers is probably the perfect example of the Complete Monster trope in Transformers fiction. He's one of the most powerful Decepticons but unlike the others he doesn't give a damn about the cause, or conquest, or even defeating the Autobots. He just wants to kill anything and everything he can, in as many sick and twisted ways as possible. He horrifies all Cybertronians, Autobot and Decepticon alike, and Megatron was wary of him that he had Shockwave alter his brain so that Overlord could never beat him. When he took over Garrus 9, he essentially turned it into a cross between a concentration camp and a gladiatorial arena, with torture, murder, and prisoners being forced to fight to the death for his own sick amusement. Nothing any other Transformers villain has done really compares to the pure evil that Overlord is guilty of.
Pharma, in the More than Meets the Eye, is s a former Autobot Medic who went evil. During the war, he saved one of the main character's lives when he was shot in the stomach, and later did a quadruple organ transfer, donating one of his organs to the process. He became the head medic at a medical facility on the planet Delphi, where he received death threats from an elite Decepticon unit who threatened to overrun the place. He began bribing them with organs from dead bots, but as the demand got higher, he began killing living ones to meet the demand. Eventually it becomes too much, and he has two other Decepticons unleash a plague on the facility to kill everyone and cover up his crime. When he's revealed, he shoots a life-support sending about 20 patients into near death states to distract everyone. He then tries to kill Ratchet twice before being thrown off the facility into a Disney Villain Death. He survives the fall and is recruited into Chief Justice Tyrest's group (Tyrest is the guy who drilled a hole in his head and is trying to destroy all Transformers he considers inferior). All the Decepticons admit to being freaked out by him, and when he finds Ratchet again, he separates Ratchet's head and spark from his body and starts torturing him. Ratchet accuses him of being a medic to prove he's the best rather than for any altruistic purposes. Pharma and Ratchet engage in a competition to prove who's the best by fixing someone. Pharma agrees and has one of his co-workers from Delphi, Ambulon, sliced in half with a chainsaw, vertically from head to crotch, and tells Ratchet to fix him. Ambulon dies in agony before the Chainsaw leaves him and later Pharma shoots Brainstorm. Even if he has a smaller body count than the likes of Overlord, and counting on what resources he had at hand, he rivals Overlord on sheer cruelty based on how his victims died.
Flatline, from the Revenge of the Fallen comics, is a sadistic Mad Doctor. Before the War he was the caretaker to protoforms, "baby" transformers, when the war broke out, he began using them as raw materials. When Arcee was almost killed, he had her telepathically bonded to two of her near-dead sisters, and tried to destroy their minds to make them into loyal soldiers. He also kidnapped Skids and Mudflap, the Transformer equivalent of children, and altered their minds, giving them brain damage, and retarding their intelligence.
Shattered Glass has Ultra Magnus. He was locked up in his universe for trying to usurp Optimus (and he got his face ripped off in addition to failing). He and several others travel to earth, where he assimilates numerous Autobots into the Junkion hordes (becoming mindless slaves and dead in all but name). After that, he proceeds to set up a plan to take rarified energon from earth with his sword and destroy it. His plan is foiled when a shield is placed around the earth but his real plan was to actually cause the death of the entire universe! He just destroyed a reality! Even the Normal Decepticons, turned against him (They wanted to be the conquerors) and were appalled when they realized the extent of the damage.
Unicron is the Bigger Bad in the Transformers verse, and exists as a multiversal singularity, traveling across dimensions in pursuit of his goal: the eradication of not only all life, but all existence from the multiverse, leaving only a swirling void centered around himself. With that goal in mind, Unicron has devoured numerous planets, scouring the life from them, made frequent attempts to destroy Cybertron (actually his brother Primus), formed cults centred about his worship, and recruited numerous beings, often against their will, to serve as his Heralds (at one point having his servants Hook, Line, and Sinker torture a future version of Galvatron until he agreed to serve Unicron). To pick a few examples: Manifesting in the G1 verse, Unicron devoured the planet Lithone, forced that world's Megatron into serving him in exchange for his life, transforming him into Galvatron, and launched an assault on Cybertron that left many Autobots and Decepticons alike dead. Appearing in a Transformers/G.I. Joe crossover, Unicron destroyed the nation of Japan, before being driven off. In the 3H timeline, he kidnapped Transformers from across the galaxy and forced them to battle to the death within his body, draining their lives from them in the process. Appearing in the Armada verse, Unicron utilized his Herald, Sideways, to escalate the Autobot/Decepticon war, and made another attempt to destroy Cybertron. When this plan backfired, Unicron bodyjacked the resurrected Megatron/Galvatron and used him to further his own ends, which once again, included the extermination of all life on Cybertron. In the Cybertron comics, Unicron showed his disregard for even his most loyal followers, killing several of them once they outlived their usefulness, while setting in motion a plan to slay all versions of Primus across the multiverse, and reduce all existence to chaos. As of his most recent appearance, in the Transformers Prime continuity, Unicron, this time depicted as the Core of the planet Earth itself, tried to wipe all human life after declaring them - essentially his own progeny - to be parasites. Immortal, omniscient, and omnicidal, Unicron remains the single greatest threat to all life in the Transformers universe.
Megatronus Prime, known to all and sundry as The Fallen. If Unicron is the Ultimate Evil, than The Fallen is his Archangel Lucifer, and like his master, exists as a single, dimension-spanning being who exists across all continuities simultaneously. Created by Primus as one of the original Thirteen Primes, The Fallen became obsessed with the notions of entropy and decay, ultimately attempting to murder his creator after betraying him to Unicron. Escaping the prison he was trapped in some seven million years before the 21st century, The Fallen visited the G1 continuity, gathered several Decepticon outcasts and kidnapped four Transformers, through which he tried to channel enough energy to break the Seal of Primus, awaken Unicron, and end all reality, boasting that his victims were but "casualties in the wars of the Gods." Defeated, The Fallen, reappeared in the film verse, where he betrayed the other Primes in the name of fuelling his own ego, and traveled world to world, destroying life and assimilating technology into himself. He successfully murdered the other Primes, but was trapped on Earth by the last of his brothers. Even then, he still managed to corrupt this universe's Megatron, turning him to evil, and starting the Autobot/Decepticon war. Eventually freed and healed by Megatron, The Fallen sank an aircraft carrier, attacked several human cities, and slaughtered a sizeable military force in his quest to claim the All Spark for himself. Sharing his master's unique nature, and his unquenchable thirst for destruction, The Fallen is precisely the kind of servant that Unicron most needs.
There are two non-Cybertronian characters from G1 to earn this status. First is King Nergill, the ruler of an Atlantis-like city that wanted to wipe out the human race and almost did the same to the Autobots. When Nergill allied himself with the Decepticons, he ultimately tuned on them when he decided he didn't need them any more. The second is Lord Chumly, a hunter who became bored with regular game and decided to hunt Optimus Prime for sport. Not only that, but Chumly kidnapped several other Autobots and put them in Death Traps to lure Optimus to him!
The human villain Dylan Gould from Dark of the Moon. He's spent his adult life aiding the Decepticons in their plans on Earth (saying they're "clients" he "inherited" from his father) and assisting them in their takeover of Chicago, which sees thousands of people gunned down in the streets, all to ensure that he'll be spared. This could just be written off as him being a Dirty Coward, but the moment that has him cross the line further is when several Autobots have been captured and Gould humbly suggests to Soundwave that they shouldn't be taking prisoners at all. When the pillars have been shut off, meaning the Decepticons' plan has been foiled and he has the option to defect to the good guys and possibly be spared, if jailed, he chooses to reactivate the pillars for no real reason. This is most sensibly interpreted as being done out of sheer spite, even at his own expense, considering the Decepticons have all their human minions killed once they cease to be useful and will likely do the same to him.