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Lemongrab definitely crossed it in "Too Old". Even though Lemongrab still left his clone alive and kept him alive on a machine, he constantly abuses him physically and verbally. Not only that, he attaches shock collars to all of their 'sons' and forces them to entertain him after he becomes a Fat Bastard. Then it gets even worse when, after Lemongrab 2 thinks his twin has finally come to his senses after hearing Lemonhope's song, Lemongrab mercilessly eats his twin (again) and orders their boys to kill Princess Bubblegum, Finn (who's too distracted by his relationship blues to care), and Lemonhope.
The Lich crosses it either when he kills Billy or when he kills Prismo, both of which were done during attempts to end all life in the universe. The latter kill is especially egregious as his victim was confused and helpless, and it's clear the Lich had no reason to kill him other than For the Evulz.
Roger the alien among us has always been somewhat amoral and sociopathic, but the behavior has been flanderized to an unsettling degree in later seasons. He may have finally crossed the line in the season 8 premiere. He becomes obsessed with Hayley, an obsession which culminates first in his shooting and injuring her then kidnapping her. He takes her to an abandoned warehouse, ties her arms & legs to the posts of an old bed and tells her he PLANS TO SKIN HER ALIVE AND WEAR THE SKIN in order to "be closer" to her. After menacing the struggling Hayley by laying out the various tools he plans to use in front of her, Roger is interrupted by Hayley's husband, Jeff Fisher who finally gets him to listen to reason. Just when it looks like everything is going to be okay, Roger takes Jeff into the bathroom, SKINS HIM ALIVE AND COMES OUT WEARING HIS SKIN thinking it will make Hayley love him as much as she loves Jeff. The episode ends with Roger realizing he's no longer obsessed with the now-horrified Hayley and shows Jeff is still alive and will be getting his skin back, but it's still going to be difficult to erase such a terrifying episode. How would Jeff & Hayley be able to live under the same roof with him ever again?
Although, Roger also crossed the line in "Virtual In-Stanity", where he kills five teenagers and brutally hits each with a limousine that he stole, simply because they didn't pay for their trip.
Barry, when he is not medicated, crosses this in "With Friends Like Steve's", which takes advantage of the family of Steve and earns the trust of Stan, then he blames Steve for making atrocities for them to go away to a correctional, then tries to kill Stan.
Stevearino, Steve's Evil Knock Off is probably the most evil character in the series, to the point where the show doesn't even try to portray his actions as Crossing the Line Twice. His atrocities are not justified and are played very seriously, and include, but are not limited to, the following: Murdered and tortured multiple cats, tried to take over the original Steve's life by kidnapping and stuffing him in a small cage, planning to kill him later, and nearly killed both Stan and Francine when they finally figured out what was going on.
One-time characters Nick and Cassandra, Francine's biological parents, either do this by abandoning her as a baby because babies can't ride first class, or when they wouldn't save Stan from being trapped under a wooden beam in his burning house because they didn't want to become a liability.
Stan realizes that maybe his in-laws are only Affably Evil and chooses Francine's adopted parents over them. As a result they were Put on a Bus.
Animaniacs: Slappy Squirrel's nemesis Walter Wolf sinks to the ultimate low in "Rest In Pieces" by faking his own death in order to turn everyone against her, including her nephew!
As of the second episode of the second season, Archer has implanted a faulty Russian mind control chip into the head of Len Trexler, head of ODIN and possibly his father. Even Archer himself feels bad about it, if it has any permanent damaging side effects.
Barry's Moral Event Horizon was probably murdering Jakov, another candidate for Archer's father.
Major Jakov's Moral Event Horizon probably came in "Dial M For Mother", when he brainwashes Archer to turn him against Malory.
Malory crosses the Moral Event Horizon in Season 2 and 3 flashbacks to Sterling's childhood: she got him drunk and taught him to gamble so she could steal his Halloween candy, and berates him for being unable to hold his liquor. She bought him a bike for his birthday, then stole it and punished him for losing it, without even telling him what she'd done.
In the season 1 finale: Admiral Zhao crosses this when he kills the moon spirit. It hadn't even done anything to him: he wanted to make a name for himself so that he would be remembered by history forever, but as Iroh told him, "History isn't always kind to its subjects."
Fire Lord Ozai started at this and just kept running, becoming the most thoroughly unsympathetic villain in the entire series. After his son disrespected one of his generals in the war room, he burned his face and sent him on a Snipe Hunt. When Zuko mouthed offto him years later, he tried to kill him with lightning. Still, villains have been redeemed from worse. What truly defines him is his utter excitement at carrying out his plan in the finale, which is to quell rebellion in the Earth Kingdom by burning the continent to nothing.
For Azula, many people felt she crossed it after she killed Aang when he was going into his Avatar state at the end of Season 2. He gets better. But if that wasn't enough, she really crossed it when it becomes clear that she truly wants to kill her brother Zuko by ordering her men to cut the rope to the moving incline. This even turns Mai and Ty Lee against her. And then at the series finale, after she embraces her insanity, she used Katara as bait so she can kill Zuko with a lighting strike. It almost works.
Sozin stabbed his best friend Roku in the back (though at least he seems to slightly regret it in the end) and ordered the annihilation of the Air Nomads.
Azulon didn't like Ozai mocking the death of Iroh's son, so he orders Ozai to kill Zuko to teach him a lesson on what it's like to lose a son. Zuko could have easily become just like that if it weren't for the guidance of Iroh.
Long Feng, the Earth Kingdom's Evil Chancellor, who brainwashes Jet and then kills him when he resists.
Hama has every reason to be sympathetic: she was stolen from her tribe, kept locked up in a prison that took every precaution to prevent her from bending, and kept there for decades. In fact, she uses this information to garner Katara's sympathy, then reveals that she escaped by bending the blood in the bodies of the guards, and is now using this same power to herd innocent civilians to a mountain where she keeps them prisoner out of blind revenge. And then she tries to forcibly teach Katara the technique, and sends her into an Heroic BSOD by using her bloodbending power to manipulate Aang and Sokka into fighting a panicking Katara. She serves as a reminder that even the most innocent people in a war can become no better than those who persecute them if pushed far enough.
Unfortunately, The Bad Guy W Ins at the end. Even in defeat. she managed to teach Katara bloodbending and fulfill her duty for vengeance.
In the episode "When Extremes Meet", Councilman Tarrlok crosses it when ennacts a curfew on a Non-Bender district of Republic City, shuts off the power to said district so he can arrest those who "break curfew" as they come out to complain, arrests the new Team Avatar for trying to help them, and to top all off, bloodbends Korra and has her exiled from Republic City and left to die alone, locked in a box.
Amon and the Equalists finally cross this by Jumping Off the Slippery Slope and launching a full-scale invasion of Republic City, complete with bombings and gas attacks.
Mr. Hiroshi Sato, who comes to not only hatefully reject his own daughter, he tries to kill her!. Needless to say, Bolin is quite right to call him a "terrible father", something Asami agrees with.
Unalaq not only crosses it in New Spiritual Age, he polevaults across it by nearly wiping out Korra and Jinora's souls. Note, Korra is his niece and Jinora is an 11 year old girl. Erasing any ambiguity of his crossing is his sadistic smile which he does these things.
Zaheer and his Red Lotus gang pretty much cross this when after Kya and Bumi are beaten from their battles with Ming Hua and Ghazan, they start attacking Tenzin savagely. If that wasn't enough, then Zaheer definitely crosses it in the season finale by poisoning Korra so that she can be forced into the Avatar State and his cohorts can kill her, thus permanently ending the Avatar. And his reaction to Korra apparently dying in her grieving father's arms is to laugh in triumph.
While Kuvira lost more and more of her apparent redeeming qualities over time, she seems to finally cross the line in Kuvira's Gambit. In the episode, Korra manages to capture Kuvira's fiancé, Baatar Jr., as Kuvira is trying to take over Republic City with her new super weapon. Baatar contacts her and tells her that Korra is threatening to take him away from her forever unless they stand down and leave. He says that he loves her, and even without Republic City they can still get married and rule the rest of the Earth Empire together. As he's speaking with her, Kuvira traces the radio signal to where he's being held by Korra. Once she has the location, Kuvira tells Baatar she'll back down and she loves him. Then she fires her super weapon on the warehouse where he's being held, willing to sacrifice him just for the chance to kill Korra. At the very least, her tone of voice and body language indicate that she feels some regret over what she's doing, but her act still proves that her empire and her ideology come before everything else.
Red Skull from The Avengers: Earth's Mightiest Heroes crosses this in season 2 where we find out that he captured and brainwashed Bucky and turned him into Winter Soldier, an emotionless killing machine and made him commit several atrocities, and did same to Doc Samson and The Falcon. Skull also infected the entire New York city with his Dust of Death, which might have possible killed many innocent civilians, just to discredit Avengers and kill Captain America, while being under guise of "Dell Rusk", the secretary of US defense.
The Batman: Joker's torture of thugs is certainly crossing this, plus what he did to Ethan. And Tony Zucco crosses it in the first ten minutes of his appearance by killing Dick Grayson's parents (before he became Robin) just because Mr. Grayson called the cops when Zucco got a bit threatening.
Batman Beyond: Return of the Joker gives the Clown Prince of Crime a rather infamous line-crossing moment: he kidnapped Robin, tortured and mind raped him for three weeks, mutilated him, "fixed" him up as Joker Jr., and invited Batman over to see his work in detail (which included home video of the boy's torture labeled as "Our Family Memories"). He then reveals that he's learned Batman's secret identity, mocks him for it (and for not having the balls to kill him after all this), and knifes him before he tosses "J.J." a spear gun to finish him off with. Also worth mentioning that this particular MEH crossing was the former page picture.
Even Mark Hamill, who had been voicing the Joker for years at this point, felt uncomfortable with the role this time.
Vilgax in Ben 10, after getting a valuable crystal, murders the entire planet for no reason. They weren't doing any harm to him, he fired on the planet for no reason other than to test just how powerful the crystal is, and shows no regrets.
Ghostfreak wanted to plunge Earth into darkness, the result of which is shown to be horrifying mutations of all living things. That was just a side effect of his plan that he didn't even care about.
Zombozo gets his by kidnapping a 10 year old girl and draining her of her energy until she's nothing but a shrivaled husk of her former self, speaking of it so lightly that it's implied it's not the first time.
In Ultimate Alien, Aggregor did this onscreen by murdering five innocent aliens in cold blood. However, he could be considered to have already crossed it when he came up with a plan ending with him murdering a newborn.
Defying Villain Decay, Zombozo crosses this by forcing Ben's mother to walk a tight rope and then lighting it on fire underneath her, intent on either burning her alive or sending her falling to her death. This pushes Gwen so far that she assumes her One-Winged Angel form and scares the living crap out of him.
Colonel Rozum imprisoning 775 aliens in a horrible underground prison for over five decades, many of whom likely did nothing wrong. Prisoner 775 for example was exiled to Earth by an alien tyrant who he was trying to save his planet from. He told Rozum this, multiple times and they didn't listen. The only good thing about this guy is his love for his family.
Prisoner 775 himself has one. Once he escapes, he intends to exact his revenge against Rozum. A viewer might not condone it, but could certainly understand his reasoning for revenge against just Rozum. Problem is, 775's targets expand to Rozum's whole family. They had absolutely nothing to do with his captivity, and he knows it. He just wants to kill them to make Rozum suffer. This may have been an intentional crossing of this on his part, as in the end he attempts Suicide by Cop, so he may have just wanted to do something that would force Ben to kill him and end his suffering along with getting his vengeance.
Charmcaster sacrificing thousands of souls, even killing Ben, Gwen, and Kevin, in order to make a Deal with the Devil that will resurrect her dead father. Though to be fair, she does have a My God, What Have I Done? and go into a Villainous BSOD when he calls her out on it, and since this is a series where alien Nazis like the Highbreed can be forgiven...
Word of God comments made online actually seem to indicate that this act is, in fact, an INVERSION of the Moral Event Horizon. Instead of being the point where Charmcaster crosses the line into irredeemability, her realization of how wrong it was due to her father's words has pushed her completely AWAY from evil, and she is thus now a True Neutral character rather than still a villain.
Captain Nemesis crosses the line in "To Catch a Falling Star" by becoming the first successful hands-on murderer in the series. He kills several people, including a random motorist just to steal his car. Even his Pet the Dog moment at the very end doesn't bring him back across the line.
In Omniverse, Malware crosses the Event Horizon when he destroys his own homeworld, Galvan B, in the finale.
Years after he crossed it in the original series, Vilgax undergoes Character Rerailment in Omniverse in And Then There Were None, where he unleashes a bomb that erases all timelines save for the one he's in now, where its version of Ben Tennyson doesn't have an Omnitrix. Essentially, he's wiped out entire worlds, killed a number of lives too great to count, including his own allies, along with the main timeline's Ben Tennyson, all to exist in a world where he can kill a Ben Tennyson who can't fight back and erase him from existence.
Tricking Jasmine into slave labor in her own lemonade stand with false promises of a pony so that when people start protesting, he can make a killing off "cruelty-free lemonade" and stick Jasmine with a $300 bill, as all the money she made was not enough to cover the expenses of professionalizing the stand at Wuncler's request. Wuncler even goes so far as to blame Jasmine for the failure of the stand!
Giving Granddad his own soul food restaurant, knowing the food is so unhealthy and addictive, the resulting crime rates would devalue neighboring Meadowlark Park enough for him to purchase and develop it cheap.
Having his grandson and his best friend set bombs in a building just to kill ONE man so that can play him up as a national hero and sell loads of merchandise. Most damning of all is that all his evil acts are for the sole purpose of money, despite being rich enough to have connections to the president.
Verminous Skumm crosses the line in almost every episode he appears, but the most infamous moments are in episodes "Mind Pollution" and "A Formula For Hate": in the former he hands out mind-affecting drugs to teenagers, causing them to go insane and is responsible for death of Linka's cousin, who died because of fatal overdose, just for the joy of ruining their lives and those of their friends and families, and in the latter he harrased an HIV positive teenager and spread lies about AIDS because the kid was an easy target and used the naivety of the townspeople about AIDS to spread hatred and take over the world. Almost every episode featuring Skumm has him crossing the MEH, but these two episodes are his most infamous Moral Event Horizon moments.
Dr. Blight crosses the line in episode "A Good Bomb is Hard to Find", where she and her future self went back in time to World War II and decided to sell nuclear bombs to HITLER (sure he was Captain Ersatz and was only referred as "The Fuhrer", but it was obvious who he was supposed to be). The Planeteers even say that this is low, even for Blight.
Zarm is one nasty work himself for two words: scorched earth
In The Cleveland Show, after Rallo almost drowns and blackmails Cleveland for it, Cleveland dunks Rallo's head in the pool twice just so Rallo would stop bossing him around.
Another candidate for this trope comes in "A Rodent Like This", Cleveland gets heavily attached to a rat and raises it as a pet. The rat winds up biting Donna and when it's confirmed the rat has rabies, Cleveland decides (at first) to not have the rat killed to test it for rabies, which basically meant he would rather let his wife die than to kill his beloved "pet". Luckily, Donna strong-arms Cleveland into agreeing with her and the doctor.
While most of the villains are willing to hurt or even outright kill children, Nurse Claiborne tries to kill Numbuhs Three and Five in cold blood!
Chester trying to feed kids to sharks. Specifically, sharks that can drive cars.
Though he isn't nowhere as near to the Devil in The Bible, The Red Guy crosses this in Part-Time Job, an episode in which Chicken was looking for employment and worked for him. The only catch was him being stripped of his feathers and almost being cooked alive in one of The Red Guy's schemes.
Courage the Cowardly Dog features Mad Dog. He abuses Bunny to the point where he buries her up to her neck in dirt when she tried to escape, he tried to drive her and Courage down with his car, and most importantly, he somehow caused Bunny's best friend Kitty to be convinced that all dogs were evil.
Just anything Katz did.
The mad scientist who sent Courage's parents to space.
Eustace maybe a Jerkass to begin with but he crossed it when he gathered up the villains to kill that "Stupid Dog"
Danny Phantom played it oddly straight with Danny's alternate future. The whole "Dark Danny creation" scene was horrific and it horrified even Vlad, which really says something. Also see Dark Danny's first appearance where he apparently kills multiple people on screen. Ax-Crazy, indeed.
In Abra Catastrophe, Crocker himself crosses this when he kidnaps and tortures Wanda to use as a source of power.
Vicky crosses it in the TV film, "Channel Chasers", when she frames Timmy for destroying his family's portrait, burning down his family's grand piano, and writing spray paint on the living room wall just to see Timmy suffering a horrible punishment from his parents. And among other things, zapping Future!Timmy's belt, causing him to fade away back into the future.
The Pixies crossed this in "School's Out: The Musical" with the revealed they'd kidnapped a child and raised him for his entire life as their own personal Tykebomb to try and take over Fairy World and the Earth.
The Anti-Fairies crossed this (and if "School's Out" wasn't their's, this certainly is for the Pixies) by using Poof as a weapon to destroy Fairy World for revenge, then choosing to destroy the Earth For the Evulz.
The whole gambit Norm pulled in "Fairy Idol". He was already a Jerkass Genie, but the way he ruins Timmy's and Chester's lives is just terrible.
Remy Buxaplenty was originally a Villainy-Free Villain who was merely jealous of Timmy's Fairies and happiness, and while he might do an underhanded tactic here and there, like when he tricked Timmy by faking friendship so his life would be too perfect and his fairies would leave him, he never resorted to violence. He went from sympathetic Lonely Rich Kid to full-on Card-Carrying Villain as of "Operation F.U.N." where he, Juandisimo and Chief Shallow Grave put Timmy and his friends through Training from Hell, running them absolutely ragged and cutting Timmy off from his fairies by having them confiscated in goldfish form. The MOH crossing really comes in when he traps them under a butterfly net, which renders them powerless with a missile ready to obliterate them. He was willing to KILL Cosmo and Wanda!
Peter Griffin went from lovable oafish jerk, to cruel sociopathic douche bag when he threw an unconscious Stewie with heavily infected head wounds underneath Lois' car so she would run him over, after weeks of no medical treatment, just so she wouldn't ask how Stewie got hurt.
In the episode "Be Careful What You Fish For", Stewie shows Brian what horrible conditions the preschool is in (kids gambling, a dead body, and a pedophile abducting a child to name a few) and asks him to talk to Emily, the teacher about it. When Brian sees her, he instantly is attracted to her and decides to date her instead. Brain then makes excuses on why the teacher isn't that bad while he ignores what Stewie has to endure in the preschool. Things get more questionable when Stewie comes home with his arm out of its socket because Emily pulled on it too hard when she pulled Stewie for back-talking to her. Brian then tries to shut Stewie up when he tries to tell Lois what is going on. In the end, Brian does call the cops on Emily, but only because she had a boyfriend already.
Stewie Griffin crosses it in the episodes "Stewie Kills Lois" and "Lois Kills Stewie". His first act is to finally kill his mother (though she was still alive afterwords), framing Peter for said murder and not caring in the slightest if his own father got sent to prison over false accusations, holds his family hostage, kills Cleveland, goes to the CIA and attacks the power grid. He then threatens to convert the world back to the Dark Ages if they didn't surrender, and commands people to throw apples at Peter. The icing of the cake of said horizon is him trying to kill Lois after she spares his life and tells her to say hello to Cleveland for him. Thankfully, it's then revealed that the episode was just a computer simulation.
While Lois is on sorta-higher ground then Peter, some may argue that she FINALLY crossed the moral line in "Go, Stewie, Go!" She notices Meg's perfectly normal boyfriend (Meg is not well liked), imagines her having sex with him (even though it IS technically legal in Rhode island because said boyfriend is 17, one year over the Age of Consent in RI, it is still creepy and dickish), gives him her "schedule", making out with him, and, when caught by Meg, she says "Uh... rape?" Yeah, Meg did not buy it.
Diane Simmons crossed it in "And Then There Were Fewer", when she kills several people, and then frames Tom Tucker, blaming him for her termination from Quahog 5 News.
Jeff (the abusive boyfriend of Quagmire's sister) from "Screams of Silence" is the most evil villain that Family Guy ever had, simply because, unlike the others above, none of his Jerkass behavior is played for laughs or justified. The moment when the show finally declares that it is impossible for him to turn back is when he knocks out Peter and Joe and attempts to strangle Quagmire on a "camping trip" (the three were actually going to kill him, although they were MUCH more justified).
Meg was shown to be no better than any of them, when she crossed it in Friends Without Benefits. After finding out that a crush of hers is gay and is attracted to her brother, Meg decides to arrange it so that Chris will have sex with him since it would be the closest thing to actually having sex with her crush. Her plan is to drug Chris and essentially have him date-raped. She never goes through with it, but the fact she considered it is enough to put her here.
In the Forest Friends episode Bush School, Anthony's father Mark crosses this when he tries to start a forest fire.
Zapp Brannigan is a grade-A Jerkass, putting himself above others and commanding suicide missions. However, some may argue that he crossed the line in The Beast With a Billion Backs. Kif dies as a result of trying to go against the Dimensional portal Yivo. After hiding in a shack with Amy and Leela, Zapp begins "reminiscing" about Kif. Zapp then SLEEPS WITH AMY. So, yeah, the jerk used a widow's husband's death.... to get laid.
If he didn't cross it then, Zapp definitely crossed it in "In Gadda Da Leela." Leela and Zapp crash on an unknown planet, alone and without supplies. Leela is trapped under a fallen tree, and delirious from lack of water, and the Earth has apparently been destroyed. Through the episode, Zapp appears to be helping Leela get better. Then we find out that the planet they crashed on is Earth, and Zapp has not only been using their current predicament for an Adam and Eve Plot-style seduction, but placed the hazards there so that he'd have a better shot at sleeping with Leela(again). The writers themselves seem to know that what Zapp did was unforgivable: they originally planned for Zapp to get off scott-free, but changed it to Leela punishing him in a rather fitting manner.
His first appearance in the series had him describe his "heroic" victory against a rampaging army of Kill-bots: He knew each Kill-bot had a preset kill-limit, so he just sent wave after wave of his own soldiers to die at their hands until every one of the robots had reached its limit and shut down.
Cobra Commander's exact crossing is hard to pin down, but his general complete disregard for any innocent life lost in the process of any of his schemes makes him a pretty unambiguous monster.
Cobra as a whole crosses this in "The Anaconda Strain" where its revealed that they plan to release a disease they have made into populations and then charge high prices for the cure that Cobra also made.
Baroness crosses this in "Going Underground". When the Mook operating Cobra's underground tunneler protests that if they keep going at the speed the drill is going at, they will cause untold damage via seismic activity, what is Baroness' response? To electrocute him and then "thank" him for his "misplaced concern", wearing a Psychotic Smirk the whole time. Then she takes up control of the drill itself.
Mindbender crosses this in the episode "The Anomaly" when, after Tunnel Rat points out that, if the Bio-Vipers he rigged to self-destruct do blow up, the explosion they cause from igniting all of the flammable materials in the sewer will blow up all of Brooklyn. This is Mindbender's response:
Murdoc finally crossed it when he kidnapped and imprisoned 2D. It wouldn't be quite so bad if not for all the horrific undertones of the drugging, incarceration and physical abuse. Granted, 2D was never the bravest or keenest individual, but he was always pretty harmless. Murdoc's brutality and the constant isolation have all but broken the singer, and it's around that point that Murdoc bridged the Moral Event Horizon.
In season 3 of Jem, Eric Raymond crossed it when he destroyed the master tapes to the music of Jacqui Benton, Jerrica's mother.note Jacqui had died in a plane crash, and they were (at that moment) the only known copies. Instead of meeting either Eric's demands (either one million dollars for the tapes or controlling interest in Starlight Music), Jerrica asked that Eric just return them, hoping that there was still some shred of humanity left in him...
Justice League: Some - if not all - of the many things that the Justice Lords (Knight Templar counterparts of the League) did in "A Better World". Lord Green Lantern attempts to kill JL Green Lantern immediately rather than disarm him or knock him out.. Also, when Lord Wonder Woman knocks League Batman down, she picks up a chunk of building rubble to finish him off. From the DVD commentary, the writers did that on purpose so that it showed that the Justice Lords while having good intentions, they ultimately are truly heroes turned villains who deserve to be defeated.
In the same episode, Lord Superman had Flash by the neck with Flash trying to appeal to any remaining good in him. Also, the Flash in their world had been killed and his death was one of the main reasons why they all became such cynical Knight Templars. However, Lord Superman, referring to moral lines he never thought he'd cross, coolly told him that "one more won't hurt", that this universe's Flash's death was just another means to an end. This contrasts him with the more-redeemable Lord Batman, who is visibly horrified when he thinks Flash's heart has stopped.
The true Start of Darkness is probably when Superman kills Lex Luthor, who was elected president. However, it is ambiguous since Luthor was about to hit the Big Red Button, probably to start a nuclear war. Afterwards, even Batman says that Supes made the right decision.
Minor villain John Dee in the episode "Only A Dream", where he "operated" on his wife in her dreams, torturing her until she died of fright, shortly after he had found out that she was planning to leave him for another man.
He didn't only try to kill his own brother's infant son. He put both his brother and said brother's infant son in a Death Trap. And the only way to save them both was Papa Wolf!Aquaman mutilating himself to do so.
Project Cadmus practically made a habit of doing this, even as they claimed they only had the good of the world at heart. Abducting metahuman children and youths to rear them as super soldiers, genetically engineering metahuman super soldiers and not only not caring that the process is unstable and so the clones inevitably die after a few weeks, but seeing that as an actual bonus, creating Doomsday, cloning Supergirl and then using the clone as a Super Soldier and assassin to keep their agenda secret, attempting to nuke an island with a kryptonite-based atomic missile in the hopes of killing off Superman and Doomsday in one shot, sending a team of supervillains to steal a magical superweapon from the Justice League's headquarters, working with freaking Lex Luthor to discredit and aggravate Superman... Yeah, Villain Has a Point that the Justice League could potentially take over the world... but, on the other hand, Cadmus does a hell of a lot more actual damage to the world than the Justice League ever does.
Monkey Fist crossed it in the fourth season. Attempting to murder and/or kidnap a child is bad enough, but he made a Deal with the Devil, or rather, a demonic monkey being in order to command it. Some could say that his Taken for Granite death was well deserved.
Warmonga and Warhok in the Grand Finale were about to kill Kim and stuff her as a trophy back to their home.
Floyd and Jolene, the villainous gators on Kissyfur, usually resort to trickery and being sneaky in order to catch Kissyfur and his friends for dinner. However, they cross the line BIG time in "The Birds and the Bears" when they steal Gus's paddlecab, which Kissyfur and three of his friends took a ride on with Miss Emmy Lou's niece Donna to prove that they were "cool", and trick Gus into following them so they can lure him into quicksand to drown and die while they use the paddlecab to catch Donna and some of the other critters for dinner. As expected, they get their just desserts after Kissyfur, Beehonie, Toot, and Miss Emmy Lou free Gus from the quicksand and soon free the captured critters and retrieve the stolen paddlecab.
Temutai, the king of the Quidahn, enslaves children and they always die in a few years. The only thing redeeming about him is the fact that he's a man of his word.
Hundun crosses this by threatening to kill Po's father just to force him to fight him. This also makes things personal for Po.
Tong Fo counts too, wanting to destroy the Valley of Peace for no apparent reason.
Ke-Pa crosses it when he temporarily killed Po.
Disney's The Legend of Tarzan has Professor Philander, rival and Evil Counterpart to Professor Porter has the same goofiness and is generally a Harmless Villain most of the time, until he went from silly to downright irredeemable when he cuts a rope where Tarzan's hanging on, sending him to fall to his apparent death. To make matters worse, rather then letting his rival check on his son-in-law, he then plans to strand him into the sea and throw Tarzan overboard just to spite his rival.
Lt. Colonel Staquait crossed it by ordering Hugo and Hooft to burn a village full of mostly innocent women and children
Dr. Rockso, one of the most popular characters in Metalocalypse, crossed this when he stole the Christmas presents Toki got for everyone, hocked them for cocaine money, then received a handjob on live television while bragging about his drug addiction, and getting Karma Houdinied because Toki experienced Diabolus Ex Machina before he could beat up Dr. Rockso.
Mr. Salacia crosses in the two-part season 4 finale, in which he brutally murders Roy Cornickelson, the elderly head of Dethklok's record label and one of the few completely good characters in the show.
Clay Puppington crossed the line in "Nature part 2" the moment he shot Orel, his own son, in the leg. How does he feel about it? No regret. Even if there are two episodes showing of how did he become the bad father to be, there is no sympathy. The shooting is effectively the point where Clay stops abusing Orel through mere neglect and misdirection and becomes actively, maliciously obsessed with destroying the boy's innocence, just so he can prove to himself that his own misery is natural and not because Clay himself is horribly twisted inside.
Nurse Gazelle gets away scot-free, but it could be argued that Adam deserved it more for starting the whole serade in the 1st place.
Jake Spidermonkey crosses it in "Poop Scoop" where he tries to save the school paper by repeatedly slandering his supposed best friend Adam with false rumors of romantic involvement with various female faculty members. What's worse is despite getting what he wanted (He only uses the newspaper as TP), he continues to slander Adam For the Lulz!
In A Canterlot Wedding, Imposter-Cadance/Queen Chrysalis crossed this by leaving Twilight trapped in the mines under Canterlot which were long forgotten so nopony would ever find her. Then taunting her with that fact, just to rub it in. And then it's revealed she did the exact same thing with the real Cadence, who is in rough shape by the time Twilight finds her. All this after letting an already-broken and regretful Twilight think all is forgiven.
The Crystal Empire does this to King Sombra immediately upon his introduction, in which it is revealed that he used Black Magic to turn the Crystal Empire into Mordor and enslave the crystal ponies who lived there. Worse still, he's given no excuse for his crimes - Nightmare Moonhad a tragic backstory, and though it doesn't remotely excuse their actions, Discord and Chrysalis were at least motivated by some component of their natures. Sombra, though, was just a regular unicorn who decided to seize power for himself even if it meant ruining the lives of everyone around him. Appropriately, the episode presents him more as a monstrous force of evil than as an actual character.
The Mane-iac ultimately proved she was irredeemable in Power Ponies by using her Hairspray Ray Of Doom on the Power Ponies to immobilize them so she can capture them and use her doomsday device on them. Even worse, when Spike/Humdrum (whom she left out due to him not having superpowers) is left to save them, she considers him useless. Fortunately, the Power Ponies' words of encouragement give him a boost of self-esteem enough for him to save them, ultimately leading to the Mane-iac's defeat.
In Twilight's Kingdom Part 2, if Lord Tirek hadn't crossed this already, he certainly did when he imprisoned Applejack, Rainbow Dash, Pinkie Pie, Fluttershy, Rarity and Spike, drained their magic, betrayed Discord, and drained him of his magic. Even earlier than that, he banishes Celestia, Luna, and Cadance to Tartarus, even though they have no magic and are powerless to do anything against him.
Interestingly inverted with Discord. In "The Return of Harmony", he almost crossed it by mind raping the main cast. A few seasons later, after taking a level in kindness, you'd think betraying all of Equestria to Tirek, including Fluttershy, would count as this, but due to his recurring remorse and reaction to Tirek betraying him not only make him sympathetic, but solidify his Heel-Face Turn.
Doof's Alternate Dimension counterpart in the movie did this as well. It was bad enough he transformed his version of Perry into a cyborg soldier, but when he took over his version of the tri-state area, he transformed it into a dystopia controlled by robots, then he really got there when he tried to feed Phineas, Ferb, Perry, Candace, and his own dimensional counterpart to a monster called a Goozim. Phineas-2 even hinted that he replaced citizens that didn't conform enough with clones.
The Drill Sergeant Nasty in "Phineas And Ferb Get Busted" who broke the spirits of the two boys simply because they're being creative by the way he lied to their parents and the way hetreated the boys. He would've been a monster, If he existed that is.
The Raccoons: Milton Midas ultimately crosses the line in "The One That Got Away" by having the pigs dump toxic waste into a beloved fishing hole which is a favorite among some of the Evergreen Forest's residents.
At the end of the second season of ReBoot, Megabyte and Hexidecimal team up with Bob to stop the Web Creature invasion. However, near the end of the episode Megabyte launches Bob into the Web and takes over Mainframe.
Recess: In the episode "Biggest Trouble Ever" the Recess Gang accidentally breaks the statue of Thaddeus T. Third III, the namesake of Third Street school, thus becoming hated by the entire town. They are labeled "The Destructive Six" and work menial jobs by Ms. Finster as a punishment. It goes too far when Mayor Fitzhugh and the city Council decide to send the gang to six separate schools, despite the fact that they broke the statue by accident and were genuinely remorseful. This shocks everybody and even Prickley and Finster think that this punishment is going too far. Luckily, Thaddeus T. Third V (grandson of Thaddeus T. Third III) called him out on this and revealed that Fitzhugh intentionally tried to break the statue when he was a child. Most of antagonists in this series are jerks, but Fitzhugh's hypocrisy is notable.
Benedict's plan, if you apply Fridge Logic to it, involved moving the sun away to have "no more summer"; would've killed all life due to extreme freezing conditions.
Dr. Slicer was already cruel and heartless, but when he demanded the cannon to be functional, he really showed his un-hidden dark side.
Mayor Jones asks the kids to come to the Cicada Festival, saying he has a plan to protect them. He actually wants photos of them being overwhelmed by swarms of bugs for tourism purposes. And he gets worse: How about kidnapping a baby from people he knew ï¿½ people he forced to leave town by threatening to hurt said baby, none the less ï¿½ and fed him lie after lie as he grew up? Oh, and he's the Freak of Crystal Cove and thus is the cause of EVERY SINGLE THING that happened to the original and current Mystery Inc.
Multiple parents in episode 5, abandoning their "spookified" kids with the assurance that the government will air-drop in food. And it was entire blocks of parents.
Brad and Judy, Fred's real parents, willingly firing a missile at a helicopter that they know Fred is inside without any regard to his safety in "The Horrible Herd."
It's hard to determine with Professor Pericles, as he excels at committing horrific acts in almost all his appearances, especially toward the end of the show.
The Nibiru Entity gets one when, after it is freed proceeds to eat Brad and Judy (after they had pledged their servitude), then Mr. E., then the rest of the town, all for power (and you can tell it's enjoying it), and it claims it will go on to devour entire galaxies. Then again, it is the most evil Annunaki ever. As such, when Scooby strikes the final blow that destroys it forever, it is VERY satisfying to see it vainly struggle to escape its fate.
The Simpsons has "Curse of the Flying Hellfish," where it's revealed that Mr. Burns and Abe Simpson are the two surviving members of their unit from World War II, and that the last surviving member would get a case of old paintings stolen from German homes during the war. When Burns tries to have Abe killed (even though the art's value would be a pittance compared to his actual wealth), Bart convinces Abe to get the case instead of letting Burns get it. When they actually arrive at the location of the case, Burns shows up too and takes the paintings at gunpoint. When Bart calls Burns a coward and an embarrassment to the name Hellfish, Burns points the gun at Bart's head. Abe says Burns can take the paintings so long as he does not hurt the boy. Burns says he would rather do both, and then kicks Bart into the empty case hard enough to send the case falling into the water, and then boats away saying "so long, Sarge, see you at the reunion in November!" In other words, Burns attempts to drown a child, (even though he could have taken the art without doing so) for no apparent reason other than that the child insulted him. Furthermore, Burns gets away with said murder attempts too. And when Abe corners Burns...
Also, in an earlier episode, "Crepes of Wrath," Bart is treated like a slave by two mean winemakers in France. At first, this is amusing, as we see Bart get what's coming to him for his brattiness. However, it fades when you see Bart sleeping with nothing so much as a blanket after reading a letter from his mother as the abuse he is suffering begins to sink in. When the winemakers finally force Bart to drink wine doctored with antifreeze, putting him in real danger of being killed or blinded, the louts sail over the moral line and all your sympathy goes to the boy, which makes his eventual escape and revenge all so sweet.
Sideshow Bob in all of his appearances tried to kill Bart, but he finally crossed the line in "Funeral for a Fiend" when he decides to get people to hate Bart while he's on trial, fake his death, and, with the help of his whole family, burn Bart alive in a coffin being pushed into a furnace. Fortunately, he is caught and, along with the rest of his family, is sentenced to 87 years in prison.
Snake tries to run over Bart when he's with Eddie and Lou in a ride along.
The Preschool Teacher's treatment of Bart, as seen in "Lisa's Sax". SHE IS THE REASON why Bart is Bart today. What makes it worse is that Bart was just 5 years old and her actions made Bart actually consider suicide. Until "The Boys Of Bummer" this was probably the worst line crossing moment in the series.
To specify, the teacher basically wrote off Bart as a lost cause because he didn't catch on to things as quickly as the other kids:
Bart: (singing) B-I-*clap*-*clap*-*clap*-O and Bingo was his name-o.
Teacher: (writing on a clip board) Extra clap; not college material.
Patty and Selma have always hated Homer and made it clear since day one that they do, but they crossed this line when they attempted to murder him when Homer and Marge were going to remarry in the later seasons. And outside of being blackmailed to pay for the wedding by Bart and Lisa, they suffer no repercussions. Outside of the Boys of Bummer and Sideshow Bob's attempt to incinerate Bart, this is the worst thing anyone in this show has done.
None of these actions compare to what the population of Springfield does to Bart in "The Boys Of Bummer". It is highly agreed amongst the fandom to be THE WORST Moral Event Horizon crossing in the series, making all the other crossing the line moments in the series look harmless. The townspeople cruelly treat Bart, simply over losing a lousy baseball game and even after Bart tries to commit suicide they don't stop abusing him. They at least apologize to Bart once Marge calls them out on their cruelty. While the townspeople appear to show regret for their actions, they still get away with it scot-free. This is still said to be the most evil Moral Event Horizon in the series.
Cecil Terwilliger absolutely crosses this in his first appearance. He conned his way into building a dam, cut back on the building materials to embezzle the funds, and then tried to kill Bart, Lisa, and his own brother in an attempt to blow up the dam, and the dam floods the town anyway because of the lackluster construction. The only reason no one was killed was because the universe dips into cartoon physics at that point. Like "Boys Of Bummer", Cecil manages to have crossed a very menacing Moral Event Horizon. And he doesn't even try to hide behind a Freudian Excuse. When Bob confronts him for Bob's role in stealing Cecil's dream of being Krusty's sidekick, Cecil makes an offhand remark how that might factor into it, but proudly declares he did it for the money.
From the episode "Postcards from the Wedge", Bart decides to manipulate Homer's newfound strictness and Marge's caring eye to try and separate them. Why? To get off of homework. While it's probably not the WORST behavior he has committed, Lisa declaring that he is "now officially a sociopath" may seal this as the point where Bart becomes no better than the rest of Springfield.
Of course, it's not like Lisa is so innocent either. There are several instances where fans consider to be a Jerk Sue to the point that she she doesn't learn from her actions or get's punished. Why? Because she's Lisa. To be specific, in "On A Clear Day, I Can't See My Sister": In retaliation for one of Bart's pranks, she takes out a restraining order against him and gleefully uses it to make his life a living hell even after he stops bugging her, forcing him to live in the yard and leaving him with permanent nerve damage.
Russ Cargill crosses it in The Movie when he tricks President Schwarzenegger into authorizing the fiery destruction and total obliteration of Springfield (and Cargill knew damn well what the options were that he gave the President).
Homer Simpson is a known Karma Houdini, and depending on who you are, there are numerous candidates for his Moral Event Horizon crossing moment. One of the earliest front runners (for him and quite possibly the series) is the episode "Homer's Enemy." In this one, Frank Grimes, a well meaning, hard working coworker who also has terrible luck, winds up receiving the horrific consequences of Homer's increasing stupidity. This involves being humiliated, demoted, and ultimately killed, for the horrible sin of not being a jackass in the town of Springfield. Homer Simpson, meanwhile, gets laughs throughout the episode, including at Grimes's funeral. Josh Weinstein, the showrunner at the time the episode was created, comments on their being a "generation gap", where the episode was panned by longtime (at the time) viewers, but subsequently liked by people who grew up with the show. For many older fans, this marks the point where the Simpsons crossed over from pointed, relevant, biting satire to plain old mean spiritedness and dead baby humor ala Family Guy, and Homer Simpson went from well intentioned but flawed and dull human being, to a criminally malfeasant monster.
South Park: Cartman crossed the line with his Evil Plan against Scott Tenorman, in six simple words below. He's done lots of evil things since, varying in their badness, but this was where most viewers decided it was no longer in-character for him to perform an act of selflessness.
Cartman: I made you eat your parents.
Butters' parents in "Jared Has Aides". After they both beat their son at the same time (and from the sounds of it, the event was pretty savage), it was generally agreed that they are, while not certifiably evil, notGood Parents. This opinion didn't change even after the episode "My Future Self And Me" when they promised never to beat him again.
They cross the line even further in "Stupid Spoiled Whore Video Playset", not only for selling Butters to Paris Hilton for $250 Million, but for scolding him when he begs not to be sold to a Spoiled Brat who treats her pets so poorly, they get Driven to Suicide.
Saddam Hussein crosses this territory when he takes the power of Canada as a tyrannical Prime Minister, forcing the family Broflovski to almost lose Ike, and later he killsKenny for no reason. All this to prove how evil he is.
Sheila Broflovski crosses this in The Movie. She tries to commit Canadian genocide and causes the World War III.
Principal Victoria crosses it in "Douche and Turd" with her actions taken on Stan for him not wanting to vote in the school presidential election. Rather than just suspending him, she sentences him to banishment from the entire town. Just because he doesn't want to vote between a douche and a turd. He eventually caves for a given reason later, but this was still extremely strict given how out of proportion it was.
The entirety of South Park America crosses it when they hounded Britney Spears to the point she was ready to shoot herself. They continue to pester her after her suicide attempt and degrade her, until finally they reveal they are 'sacrificing her to the harvest' in a twisted Pagan Ritual and proceed to kill her by making her have a heart attack from everyone taking her picture.
In Sponge Bob Square Pants, Plankton crosses the Moral Event Horizon in the movie. He stole King Neptune's crown, frames Mr. Krabs for it, enslaves Bikini Bottom, shows excitement and joy when Mr. Krabs is going to be executed and sends Dennis to kill SpongeBob and Patrick.
Of course, Mr. Krabs isn't exactly a saint himself considering how greedy he is, particularly during the Seasonal Rot. In "One Coarse Meal" he establishes his Seasonal Rot personality when he exploits Plankton's fear of whales by dressing as his daughter, Pearl, threatening to eat him. Krabs harasses Plankton so much, the latter eventually becomes suicidal. After Krabs found out that Plankton was ready to commit suicide, he simply laughed at Plankton's misery. Worst of all, apart from temporarily being hung upside down & forced to watch a mime's preformance after Plankton finds out, he doesn't receive any comeuppance for this. And of this is for fun.
In "Cephalopod Lodge", the members of the lodge crossed this when they kicked Squidward out just because they thought SpongeBob and Patrick were following him. It's made even worse when you realize the lodge was one of the things that made Squidward genuinely happy in his whole miserable life.
In the episode "Demolition Doofus", Mrs. Puff crosses it with her attempt to kill SpongeBob in a monster truck derby when she got tired of his antics, even stepping in herself when he managed to prove himself a natural at it. Not just injure him, but kill him outright!
The Ugly Barnacle could've crossed this by killing everyone in his debut story... which wasn't exactly a long story anyways.
Zygeerian Keeper Arguss wants to make sure Obi-Wan does not try to attempt anything against him, by dropping some Togrutan prisoners down a bottomless gorge. And then later, when the Republic has arrived to liberate the surviving slaves, he sets the system to dump the slaves into the abyss, and then smashes the panel to prevent Obi-Wan from saving them.
Osi Sobek crosses this when he gleefully shoots a captured Clone Trooper in the face.
The Trandoshan Slavers, what with their capturing and hunting Jedi Padawans, as in, teenagers, for sport.
Pong Krell, when he cruelly tricks 501st Legion and 212th Attack Battalion Clone Troopers into killing each-other before Rex realizes that they're being duped. Worst of all, among the clones killed include Waxer.
Darth Maul takes a running leap over this in the fourth season finale. While most would argue that Maul had already crossed it when he killed Qui-Gonn, Qui-Gonn is a Jedi, Maul is a Sith. Nothing personal there. What is personal, however, is murdering a bunch of innocent people, including children, just to lure Obi-Wan for the sole sake of revenge.
Teenage Mutant Ninja Turtles 2003: Shredder's personal Moral Event Horizon comes in the episode "Mission of Gravity", when he orders his lieutenants to steal the Triceraton engine holding the city of Beijing aloft miles above the earth, despite the fact that doing so would naturally doom the millions trapped within it. It was possible to remove the engine without killing anyone, but he didn't care.
When Leonardo saves Karai from falling to her death, she stabs him from behind with his own katana when he would have let her go in the spaceship anyway. This is one of the things that leads him to tell her she is just as bad as the Shredder. It can be argued that every decent thing she did after that was just self-preservation.
Technically in that instance, Shredder punched him into the blade, which Karai held at that moment. If you look at it for a split second she's visibly shocked by what just happened. So in that case it's TECHNICALLY an accident.
Karai's alternate future self from "Same As It Never Was" aided Shredder and served him as his right-hand, even when Shredder took over the entire planet, turning it into a massive hellhole, enslaved humans and Utroms, planned to invade other worlds and reduced much of Earth's population and killed everyone who opposed him in brutal ways. Yet she still aided him and never even expressed any remorse for any of this, while stating that she just follows her duties, but at this point this argument to defent herself is MEANINGLESS. She is just as accountable for this as the Shredder. Karai from present at least questioned his acts and even went against him in "Mission Of Gravity" and pulled a Heel-Face Turn during the Turtles Forever, when she found out that Shredder was attempting to erase the entire universe.
In his backstory, he burns Splinter's house and kills his family, simply because Splinter married the woman he was in love with.
In the first season, he allies himself with the Kraang, making a deal with them that would result in everyone on the Earth dead or mutated, just so he could get Splinter out in the open.
First season finale: It's revealed that Shredder didn't kill Splinter's daughter, Miwa, he kidnapped her and raised her as his own - Karai. Meanwhile, he fed Karai a lie that Splinter killed her mother, giving her a vendetta against Splinter as well. He's been using her as a pawn for revenge her ENTIRE LIFE, and she doesn't have a clue. And he doesn't hesitate to sic her on Splinter when she comes to help Shredder.
And when Karai learns the truth and asks him about it, he casually tells her yes then has her locked away in the dungeon without a second thought.
In ThunderCats (2011), the act that cements Mumm-Ra's evil happened in the backstory when, on the behest of the Ancient Spirits of Evil, he destroyed the Plundaar star system which was inhabited by billions to create the metal needed to forge the Sword of Plundaar. It even acted as one in-universe. This heinous act convinced Leo that Mumm-Ra was not the visionary bringer of order to the universe that Leo thought he was but a monster that needed to be stopped.
In Transformers Prime, Megatron, is revealed to have crossed the line multiple times back on Cybertron. Aside from instigating the very war itself, he developed a cybonic plague and a bioweapon out of toxic Energon, both of which wiped out most of the Autobots (and lord knows how many innocent Cybertronians) gradually turning the entire planet into the graveyard it is today. However, he arguably crosses the definitive line in "One Shall Fall", when he shoots Bumblebee with Dark Energon, nearly killing Raf in the process. While Megatron was unaware of Raf's presence at the time, his reaction later on establishes he has no problem inflicting off-world casualties. For Optimus, this was the last straw, as he saw no alternative than to destroy Megatron once this happened.
Airachnid crossed it millennia ago when she tortured Arcee and murdered Tailgate. Beyond that, we learn she's spent who-knows-how-many years hunting species and committing genocide. For fun. Whether this started before or after Tailgate's murder is unclear.
Silas crosses it when he works with Airachnid to kidnap June Darby and then shows he's willing to let Airachnid kill her and Jack, dismissing them as collateral damage.
In "Evolution", upon discovering Predaking's transformation, Megatron and his command staff plan the eradication of the newborn Predacon clones, before said clones are given the chance to turn on the Decepticons. Starscream suggests they set it up so the Autobots would look entirely responsible and turn Predaking's anger towards them instead of his creators. Even Shockwave, the creator of the clones, is coldly in favor of the idea.
Hard to imagine that a show withsillyvillains like T.U.F.F. Puppy have someone crossing the line, rather it's not a villain but a former agent of T.U.F.F, Jack Rabbit. After he's been revealed that he betrayed T.U.F.F and lying to Kitty doesn't sound bad but his reason for his betrayal: Money! That's right, no Start of Darkness, no sad backstory, it was all due to his Greed.
In Season 4 of Winx Club, Ogron, the leader of the Wizards of the Black Circle, got to this dreaded point of no return. After the Wizards pull their I Surrender, Suckers plan on the Earth fairies, Ogron tears open a portal to a nightmarish dimension to imprison all the fairies there. Aisha's boyfriend Nabu does an Heroic Sacrifice to close the dimensional gap, but loses his life in the process... and when the Winx try to use the Gift of Life to bring Nabu back, Ogron snatches it and casually casts it away, laughing all the way. This single act caused the distraught Aisha to join the Fairies of Revenge and separate herself from the Winx Club, and marked the point where the Wizards of the Black Circle sunk to a low no other Winx Club villain ever reached.
In W.I.T.C.H., you have a number of villains, but one who could easily stand out is Nerissa. Where Prince Phobos was already a Smug Snake, Nerissa started out as a normal woman, no, Guardian. But, she got a little too attached to the Heart of Candracar and was forced to hand it over to her dear friend Cassidy. When Cassidy refused to return it, Nerissa murdered her. She would spend the next generation plotting Phobos' downfall so she can take over, armed with as many Hearts as possible, going on a dog-kicking spree "for the good of the universe".
Pietro had his when he had a train derailed for the purpose of pretending to save people, and upon finding out that if they fail hundreds will die, including himself, he runs away and leaves those trapped, and potentially the entire town, to die. He knew his Brotherhood friends and his sister were there, and he left them there anyway;
Toad stole Kurt's image inducer — not that bad, until you remember that he purposely chose to do it while Kurt was meeting with his girlfriend Amanda's parents, ruining his chance at making them like him.
Magneto had it when he had his own daughter Wanda mindraped, just so she'd stay away from him. Said mindrape took place via replacing her memories of him abandoning her in a mental insitution, which is what made her become a Dark Magical Girl in the first place, into loving memories that made her forget his abuse;
Mystique when she tried to kill the New Recruits, for no reason other than them being unable to fight Magneto. And she did it again via kidnapping Scott, stealing his Power Limiter aka his ruby quartz glasses, and leaving him stranded in the middle of nowhere
Duncan Matthews and his pals went from Jerk Jocks, to physically assaulting Scott unprovokedly after he was outted as a mutant, to post-graduation attempted mutant murder with mining machinery. That last one got them sentenced to life in prison. Duh.
There was also the soft-drink manufacturer who unknowingly laced his drink with mutant poison. You can tell he's genuinely surprised... then Mood Whiplash kicks in and he figures he can make a lot of money off the discovery.
Apocalypse was planning on using his machine to turn the populace into mutants... But even he notes that this may kill much of the human population, due to their bodies not being able to take it. He still was willing to go through it.
In the Redhill story from Animated Tales of the World, the Adviser crosses this when he orders the guards to kill Nadine and his grandmother, even though he had already successfully convinced the Rajah that he shouldn't take advice from a young boy such as Nadine.
Bluto from the Popeye cartoons crosses the line in almost every episode whenever he harms Olive Oyl after she rejects him, which includes kicking Olive off a runaway roller coaster in Abusement Park and trapping Olive in a straight jacket and then approach her menacingly in The Royal Four Flusher.