Dahlia Hawthorne from Trials and Tribulations crossed the Moral Event Horizon years before her first appearance in the game, but easily makes up for it by her actions in each episode she's in. Her first act chronically was to manipulate her slow boyfriend Terry Fawles into taking part in a fake kidnapping organised by herself and her step-sister to get a large diamond from her father, who she hated for taking her away from her home. Once the "swap" was made Dahlia took the diamond and jumped in the river, while her step-sister arrested Terry for her "murder" and testifying that Terry threw Dahlia to her "death" on Dahlia's instruction. When the guilt was too much for said sister and she called Terry to tell him the truth, she murdered her and placed her in the boot of Terry's stolen car to frame him for another murder he didn't commit. But despite all of that he was still in love with her, which she manipulated by telling him years before that if he ever doubted her love for him for a second that he was to drink a vial of poison, which he did just as Mia Fey was about to prove his innocence of every crime he had been accused of.
After said incident she was investigated by Mia's boyfriend Diego Armando into the crimes she was suspected of, but she outsmarted him by agreeing to a meeting and poisoning his coffee with a vial she stole from her next boyfriend, who true to form was being manipulated by her as well. The poison almost killed Diego, putting him into a several-year long coma and severely damaging his nervous system. The worst part is that this action was completely unnecessary, as with Terry and her step-sister dead there was no evidence that could prove she was guilty.
She then took the poison vial she kept hidden in her necklace and gave it to the first person she could manipulate, which happened to be Phoenix Wright. She tricked him into thinking it was a gift so he would take it with him, preventing the police from finding it and linking her to Diego's poisoning. However, when Phoenix began talking about his new girlfriend she manipulated her twin sister Isis into trying to get it back, which over the next six months led to her falling in love with Phoenix as well. Then without consulting Isis Dahlia decided to speed things up by poisoning Phoenix's cold medicine and taking the necklace off his corpse, only to be foiled by her ex-boyfriend taking the pill bottle off of him and telling Phoenix that Dahlia was manipulating him. Out of vengeance, Dahlia pushed him into a live wire and electrocuted him, before trying to frame Phoenix for the crime and see him executed, which Mia stopped by daring her to take one of the poisoned pills.
After the events of the above case Dahlia was sentenced to death for all of her crimes, and vowed to make Mia Fey suffer greatly for exposing her crimes, a vow which was cut short by Mia's death at the hands of Redd White. But a few months before her execution Dahlia met her long-lost mother Morgan Fey, and the two hatched a scheme to kill Mia's sister Maya to make Morgan's third daughter Pearl become the matriach of the Fey family, while Dahlia went on purely to cause Mia's spirit more pain. Taking steps to manipulate her twin again to help with the murder, she came back from the dead to possess Misty Fey (albeit due to Misty's desire to protect Pearl) and attempted to murder Maya, being stopped only by the intervention of Godot who killed her host. But despite this she became convinced that she had forced Maya to murder her own mother and caused her to commit suicide, something she took great pleasure in announcing and rubbing Phoenix's face in.
She's also a giant hypocrite; she shows nothing but disgust towards Morgan Fey because of the "petty revenge" that she was after, but at the same time, Dahlia was going along with Morgan's plan specifically in order to get her own revenge on Mia.
In Apollo Justice, Kristoph Gavin. When one of his clients summarily dismissed him after a game of poker, he took it rather personally. So personally, in fact, that he didn't just ensure the man's case was lost - he got the lawyer defending him disbarred, using some evidence that he had ordered forged for his own trial, then tipping off the prosecution that it was a forgery. This, on its own, would only qualify him as a Magnificent Bastard, but there is more to the story. There were several loose ends to clear up, including the defendant's disappearance. First, he staged an attempted kidnapping of the forger - a sheltered, timid twelve-year-old girl - and then, when it left her too traumatized to go outside, he stopped by and gave her a good luck charm of a bottle of clear nail polish... which had a painful, extremely virulent, slow-acting poison in it. Kristoph had noticed her habit of biting her nails when nervous, so surely that would take care of her if she was ever pressured about what he'd had her make. He then meticulously kept tabs on everyone who had been involved with the case for seven whole years to make sure nothing went wrong - including befriending the man whose career he had ruined. And if that man hadn't been Phoenix Wright, he probably would have gotten away with all of it.
What really seals Kristoph's status as a monster is that he is like a combination of Dahlia Hawthorne and Quercus Alba: he cares nothing for anyone else but himself, he sees other people only as tools, and he will carry out whatever atrocious deeds he feels serve his own interest.
This is shown in perfect detail with that poker game that set all his events off. He forged evidence that HE requested made? He later used it to disbar Phoenix but its original purpose was to win the trial. His opponent in that trial was his KID BROTHER. Yeah. His own little brother was the prosecutor and he was so intent to win that he would forge evidence to beat his own brother in his brother's first trial ever.
Matt Engarde may well be the biggest Complete Monster in the Ace Attorney series. There is no motive given for the atrocities he committed (outside the murder), leading some to believe he did it For the Evulz. These atrocities are throwing away his manager, Celeste Inpax (who he knew loved him), like a used tissue, and when she fell for his rival and got engaged, he told him that he was previously intimate with her in the hopes this would cause him to call off the wedding. It worked. Celeste Inpax was so heartbroken that she committed suicide. He doesn't just stop there. When he found out his rival now plots with his new manager, Adrian Andrews, against him, he hired Shelley de Killer to kill his rival, but still puts in extra plans to blackmail him. Oh, by the way, he also laughed when Shelley de Killer kidnapped Maya so Phoenix Wright would become his attorney and get him acquitted for his crimes. Then he let Phoenix PIN the blame on Adrian, already an insecure, miserable woman. The knocker for this is because Engarde probably knew about Adrian's past and still did it for extra lulz.
Additionally, Manfred von Karma. As a prosecutor, he had a flawless, decades-long win streak, until defense attorney Gregory Edgeworth managed to (rightfully) have von Karma penalized for his illegal workings in the courtroom. Although Gregory still lost the case, von Karma was enraged by the black mark on his previously-perfect record. Unfortunately, right after the end of the case, an earthquake hit the building. It trapped Gregory Edgeworth, his nine-year-old son Miles, and a bailiff inside an elevator. As oxygen was beginning to run out, the bailiff panicked and attacked Gregory. Miles threw a piece of evidence from the trial at them to get them to stop fighting, which turned out to be a gun. The gun fired once, a terrible scream was heard, and then Miles passed out from lack of oxygen. Unbeknownst to the people in the elevator, von Karma had come across them. The bullet from the gun shot through the glass and hit his shoulder, causing him to scream. In revenge for the black mark against his record, von Karma picked up the gun and shot the unconscious Gregory in the heart. Afterwards, von Karma took Miles under his wing and, for fifteen years, raised Miles to be the antithesis of everything that Gregory Edgeworth was, twisting the boy to fit his views and become a ruthless, cold-hearted prosecutor who cared for nothing but getting a guilty verdict every time. Finally, on the very last day before the Gregory Edgeworth murder case was closed forever, von Karma set up a murder and pinned the blame on Miles Edgeworth, intending to have his own foster-son executed for a murder he didn't commit. Von Karma also planned it out so that Miles would be convinced that he himself was the one who had killed his father and would confess to and be charged with that murder as well.
Redd White does not care how many people commit suicide after he reveals their secrets or get ostracized because of hurtful information that isn't accurate, as long as he gets money to buy gems and women he can show off. He doesn't care whether the woman investigating him is a great mentor, sister, advocate, etc.; he'll still watch to see if she gets too much on him and kill her if she does. He also doesn't care how innocent the people he's setting up to take the fall for his actions are. Redd White may be one of the most hated non-final villains in the series.
Investigations has Quercus Alba, the head of a large smuggling ring. The other criminals of the game (except Lance) answer to this guy. He is the one who ordered the deaths of Cece Yew and Byrne Faraday and killed Mask*DeMasque II and Manny Coachen with his own hands (well, with a statue and a knife, but you know what I mean). His reasons for these killings are as follows: Cece Yew was a witness who needed to be silenced, Byrne Faraday was a member of the Yatagarasu whose goal it was to expose his operation, DeMasque II was going to steal his fake statue and, with it, some plates used for a large-scale counterfeiting operation, and Manny Coachen because he ordered DeMasque II to steal it. Manny ordered the theft because he was planning to betray and usurp Alba and was unaware that the fake statue had been switched with the real one. He almost gets away with it due to Diplomatic Impunity, too. Also, confronting him and getting him to admit to his crimes is...difficult. And lengthy.
The Dragon for Alba, Calisto Yew, isn't very nice either. She arguably takes being an Amoral Attorney to even greater lengths than Gavin. She had no good reason to shoot at Edgeworth at the end of case 4. She kills people and laughs about it. Most cases in the series focus on one victim — Calisto killed two people in I-4, one of whom was her "coworker" in the Yatagarasu and the other was her own defendant. She faked being the sister of the woman Manny Coachen killed, but then helped Alba try to cover up Coachen's murder. She's The Mole on two separate occasions. She's quite a fitting dragon to one of the biggest monsters in the series.
Investigations 2 has Bansai Ichiyanagi. Sided with the Bigger Bad of the story to assassinate the real president of Zheng Fa years ago and killed the reporter of that witnessed the murder. Later on, he killed the reporter's girlfriend and framed Kay for it. He also gave Manfred Von Karma the only penalty he ever got because he got caught forging evidence, despite the fact that he helped Von Karma with the evidence and is thus responsible for the DL-6 incident that shortly followed. Additionally, he blackmails many characters in the series to work with him if they value their lives which include tampering with the crime scene and hiding crucial evidence. He even blackmails the judge Mikagami to get her to work against Edgeworth because he kidnapped her adoptive son.And let's not get started with the traumahe puts his son, Yumihiko, through the entire game, who he constantly berates and tells in the worse way possible that all his achievements was because of his connections (and knowing that Bansai has a lot of connections with shady folks, one can only imagine what would have happened if they didn't comply) and not from Yumihiko's hard work. Later on, he kidnaps his own son (but, to be fair, he was aiming for Mikagami's son), and when Yumihiko finally snaps out of his Heroic BSOD thanks to Edgeworth, when he tries to confront his father, he proceeds to humiliate his son even more in court. Finally, the game implies that he murdered his own wife, but the circumstances around that remains unknown. This guy here is definitely worse than Von Karma.
School Days: Taisuke evolves into one come Cross Days. He finds out that there is a new "girl" that Makoto is hanging out with named Yuu and stalks "her" until it is revealed that the person that Makoto is banging is Yuki. He blackmails Yuki and forces him as his sex slave, and if he told anyone about it, he would show all the pictures in the school of the type of person he really is, effectively ruining his reputation and thus ruining Yuki's chance of getting Makoto and Kotonoha hooked up together again. He then brings his other friends in on the deal and gang bangs Yuki constantly, and, in one of the endings, it sends Yuki over the edge.
The orphanage leader from Miyo Takano's past was quite a despicable bastard. He kept children in line by fear, whipping the disobedient ones while others could hear them. Then there's the scene where one boy accidentally spills his ink, so the leader forces the boy to eat it. It's his treatment that helped turn Miyo Takano into a delusionalpsycho herself.
Teppei and his partner-in-crime, Rina, count as well. Teppei takes pleasure in abusing Satoko and Satoshi and forces Satoko to buy him bags of alcohol despite her being underage while he gambles. Rina, Teppei's whore, tricks men in debt to Teppei to accept her help, getting indebted to her with interest. She tricks Rena's father into a badger game in order to blackmail him for money(said to have done the same to countless men before him) and tries to strangle Rena herself. Rena ends up getting the upper hand and beating Rina into gory pulp.
Miyo Takano's tragic past doesn't stray her away from being truly villainous. She still murders about 2,000 of her own villagers with the Hinawizawa Syndrome and kills Rika and her friends in The Massacre Chapter. Her true line of villainy happens when she asks Satako which was green: broccoli or cauliflower. It has been implied that Satoko is color-blind, has no idea either and guesses right. Takano kills Satako anyway, which is not even necessary to the plot, so it's conclusible she does it for fun. She's also shown to insert her own boyfriend with the Syndrome, killing him, before faking her own death in the other worlds. On top of this all she has murdered Rika's parents in the backstory, because they would intervene in her plans. She also has a weird obsession with torture weapons and is implied to try to perform a live vivisection on Satako. Many fans feel experience her eventual Villainous Breakdown as greatly satisfying.
The Yamainu can also be seen as this since they are willing to murder innocent children and back up Takano's genocidal plans; especially Okonogi for telling Takano to commit suicide by shooting herself.
Bernkastel is the worst offender. Starting at the end of arc 4, she ends up with quite a laundry list of horrendous acts. Conspiring with Lambdadelta so that Beatrice is their own personal plaything is the least of these. In arc 4 alone, she sets Ange up to have her life screwed over by the irredeemably fucked up at that point Eva and finds the result amusing. And for all her trouble, how does Ange get rewarded? By being RIPPED APART and presumably left that way. Bern and Lambda even JOKE about it. To top it off, Bern's only regret is that she lost a big advantage.
She ramps this up in Arc 5. She invokes Decoy Protagonist and Hijacked by Ganon when Battler refuses to play along and pulls out an actual Expy of Rika named Erika Furudo to serve as the new Protagonist. And Erika herself is a Bitch in Sheep's Clothing who immediately points the finger at Natsuhi, who currently is not having a good day. Which gets WORSE, now that being implicated for the murders brings her sister-in-law Eva's wrath down on her as the victims include her husband and Morality Chain, Hideyoshi. She also leads Battler to the Gold Room, which makes him the next successor, adding fuel to the fire regarding his family's stake in the murders. And manages to successfully disprove Beatrice, with Battler unable to help since he's too busy being IMPALED. Then Lambda makes Battler the Golden Sorcerer, effectively cementing him in the role of the 'villian'. And you know what the best moment of this is? Battler and Beatrice's furniture give Erika and her minions the beatdown they deserve. Naturally, Bern revels in this, until the point where Erika gets her ass kicked. She even has a smug 'Oh, did I piss you off?' look on her face at times.
Eva-Beatrice. Sure, she's cute and all that with her Ambiguous Innocence, but admit it, she's also pretty scary and nasty as well. The main reason why she qualifies is that she loves torturing and killing other people over and over again in the most gruesome and embarassing ways possible. So she kills one person in a brutal and playful way, big deal. Then she revives that person just so she can kill him/her again. Repeatedly. Until she's completely and utterly satisfied. When Beatrice tries to remind her that she told her not to be too cruel with her murders, Eva-Beatrice just tells her off. One of her cruelest acts would be breaking her promise to Eva not to kill Hideyoshi because she's annoyed by his attempts to knock some sense into her after she orders the Siestas to kill Rudolf and Kyrie. Yeah, this girl's a monster.
At least, until EP 8, where she reveals that she actually cared for Ange despite hating her. Cue Love Redeems.
But does this really count for the people that she killed before this was revealed?
Erika. What starts out as a seeming lack of tact turns out to be an obsession with intellectually destroying everyone around her and driving them to submission - a skill she first unleashes on Maria. She takes absolute, sadistic glee in her attempts to deny Beatrice and to "prove" the crying, helpless Natsuhi is the culprit, tortures her underlings when they fail her, murders the six victims of the first twilight in EP 5 by severing their heads (including Maria, whom she kills after disposing Rosa in front of her), and announces that she committed the murders with a pleasant smile. And she gets even worse in EP 6, where, among other things, she traps Battler's consciousness in a logic error and nearly forces him to marry her, fully intending to repeatedly rape his helpless body and make him watch from the logic error hell.
Black Battler from Ougon Musou Kyoku CROSS. He's a negative palette swap of regular Battler who lacks the good-guy depths that his real counterpart does, not only talking about killing everyone on Rokkenjima but also suggesting to female characters that he's going to rape them, and he "tastes" male characters. He has a wicked habit of killing anyone he feels like killing, strictly For the Evulz, and laughing about it, and he's a master of Dissonant Serenity.
Phantom Of Inferno has the character Scythe Master, in the first ten minutes it is established that he used a combination of drugs and hypnosys to erase the main character's memories, why? Because saw his potential as an assassin wanted to test a new theory. He then leaves the main character to train for months in the Mojave desert with no contact from the outside world. If this wasn't bad enough this was the second experiment. The First person he experimented on he rapes, brainwashes, trains as a serial killer and treats like property, oh and the girl was twelve when he started this. A third person he experiments on he doesn't drug or rape, but he does basiclly lie to her about events, encourages her growing hatred and turns her full Yandere just to mess with the main character. Oh and while he is doing this with her, he is repeating his first experiment, six times over, just in case.
In Tsukihime this falls to either Nero Chaos or Makihasa Tohno. The first one's Establishing Character Moment is killing and eating over 100 people because he can without even paying the slightest attention to doing so. No sympathetic aspects are ever attached to him and his entire reason for living is simply living long enough to see what kind of horrible monster he'll end up becoming. The second one raped a girl for years starting at the age of nine or ten, tops. Plus murdered the entire Nanaya clan except Shiki for no apparent pressing reason. Some other bad stuff concerning his treatment of Shiki, SHIKI and Akiha as well. Oh, and finally, the fact that he randomly murdered the family pets occasionally and nobody appears to find this a strange event for him in retrospect is a pretty clear indication that he was not a model citizen before dying at the start of the game.