Why not tell us something we DON'T already know, Joker?
Eric in A God Somewhere crosses the Moral Event Horizon for most when he rapes his sister-in-law and cripples his brother. But the narrator, his former best friend Sam, notes that throughout the series of ensuing mass murders, he was still ambivalent about Eric. He finally crosses the Moral Event Horizon in Sam's eyes when the military unit with whom Sam had been embedded as a reporter sneak up on the cave where the two of them are talking, and Eric slaughters them right in front of him.
The Purple Man started out as a low-grade Daredevil villain. Then came Alias (not to be confused with the TV show), in which he humiliated, abused, and tortured Jessica Jones in every non-rape way available. But he does rape people as well. Just in case there was any doubt he's a total scum bag. He kidnapped women and raped them in front of her as a way to mock her for being an ineffective superhero.
Talia Al Ghul proves she's just as much a monster as her father when she allows her own son Damian to be brutally slain by his own clone. She does have the decency to shed a few tears afterwards...which she dismisses as a moment of weakness.
Even this ante is upped when you realize how many times Ra's has forgiven Talia's much deeper and constant betrayals. In her current spoiled brat persona, she cannot give her son such forgiveness even once, for being somewhat like the man she fancied.
Even Ra's notes, with some twisted fatherly pride, that she has at last become a monster.
In Batman: Year One, Commissioner Loeb starts out as a typical corrupt cop, nasty in his own way but not extraordinarily bad. His true crossing of the horizon is when he firebombs an apartment building to catch/kill Batman, callously dismissing Gordon's concerns about the derelicts living inside (at least one of whom was killed).
Nekron of "Blackest Night" is an Eldritch Abomination who is said to be beyond good and evil. And at first he seemed like that. Raising the dead as foot soldiers and sending them to terrorize their loved ones? That's pretty ruthless, but not exactly personal. Turning all the resurrected heroes into Black Lanterns against their will? More evil, but considering that he's the literal embodiment of death, it's understandable. But once it's revealed that the living heroes who are turned into Black Lanterns are CONSCIOUS and forced to watch as their possessed body is used to attack their loved ones, all the while slowly wasting away until they turn into a Black Lantern for real, Nekron goes from force of nature to Complete Monster.
Recent developments in Blackest Night: The Flash have fans of Captain Boomerang the Younger wailing and gnashing their teeth. Owen's moral compass is shaky at best. Upon learning of his heritage, he joined the Rogues with minimal prompting and showed very little remorse or hesitation when it came to killing people in certain situations. Those situations being fights. After leaving the Rogues, his character was more developed as a screw up who's looking for a family, and whoever's giving him affection and approval (or at least a group to hang with) gets his loyalty. He's had ice cream nights with Supergirl for pity's sake, and has always been more or less shown to be a pretty okay guy, for an occasional assassin. So in the latest installment of the series, we learn that baby-Boomer was feeding criminals to his zombie-fied father to somehow bring him back to life. Okay, a little creepy, but it's not so ba— Oh wait, no he fed zombie!Boomer innocent women and little kids too apparently! Thus making a formerly endearing, kinda sweet character...so full of squick.
Another Captain America baddie who fits this trope is the Scarecrow (not to be confused with the Batman villain). While he was always a criminal who enjoyed scaring and robbing people, when he's captured by Captain America he goes completely over the edge and becomes a sadistic murderer, impaling innocent people with his pitchfork as a means of trying to draw Cap out to stop him. He gets the attention of Ghost Rider instead, and ends up impaled on his own pitchfork in the ensuing battle. Things went further downhill when the Scarecrow was revived as an undead zombie with the ability to cause fear in his enemies.
Cheshire, a Psycho for Hire in The DCU, crossed the line when she nuked Qurac as part of her plan to extort the nations of the world. Gail Simone outright called Cheshire a monster due to this act and rarely writes her as anything else.
Cheshire:I decided to make a point. America? No, that'd incur too much wrath. And I like shopping there. Russia? They're suffering enough as is. England? Maybe we're all sick of hearing of Di and Fergie, but maybe not...I COULD blow up a little out of the way island, but that wouldn't get the point across...Then I realize...the terrorist capital of the world. That little eyesore, Qurac. Oh, sure, you'll all rage and complain, but inside? You'll be THANKING me.
Jodafra from the Eighth Doctor Doctor Who Magazine comics is initially introduced as a roguish trickster who is nowhere near as bad as some of his relatives. Then in his second appearance he tries to feed a bunch of little children to a monster in exchange for mystic power and, when his own niece objects to this, gives her a No-Holds-Barred Beatdown with his walking stick that leaves the Doctor, who's seen some pretty ugly stuff, shocked when he sees the state of her. Unfortunately, thanks to an Aborted Arc, we never got to see him get his comeuppance.
In Gotham City Sirens #20, Harley Quinn crosses the line. During her bid to kill the Joker in Arkham Asylum she murders an innocent guard via explosive in the face. Even worse, she acknowledges that the guard is an innocent man, but she is too full of rage to care, and the guard is too intelligent to be distracted by other means.
The Guardians cross the line in the early issues of the DC Nu reboot of Green Lantern when they wipe out Ganthet's emotions to turn him into a cold emotionless Guardian like them. Kyle is understandably outraged by Ganthet's emotional lobotomy and vows to fight the Guardians.
In the DC miniseries Identity Crisis the previously bumbling and mediocre villain Dr. Light is revealed to have raped the Elongated Man's wife, Sue Dibny on the JLA Satellite years earlier. What he was doing there in the first place isn't revealed, nor was it the first time he'd done it. Unfortunately, that wasn't the worst thing that happened to Sue in the book.
In Infinite Crisis, Superboy-Prime was presented as a confused teenager with powers he couldn't control lashing out at people who didn't understand him... until he lost it and killed some C-List Fodder, whereupon he turned into a near-demonic Card-Carrying Villain, excusing his actions by claiming to be "better than those losers". Superboy-Prime picked a fight with the Conner Kent Superboy and got jumped by the full Teen Titans roster. He lost control of his own strength when he fought back, killing some of them and provoking a My God, What Have I Done? moment. At that point the Flashes drag him into the Speed Force, where they imprison him for an unknown length of time. When next we see him, he threatens to kill even his friends and allies to get what he wants. And he's terrified of all Flashes. DC has never explained what they did to him.
If he didn't cross it there, he certainly crossed it when he killed Earth-2 Superman. While the above is rather monstrous, at the very least Prime had gone through a lot of hardships and had been convinced by Alex Luthor Jr that they were in the wrong, not to mentioned he was still trying to get back Earth-Prime. Earth-2 Superman wasn't one of the heroes that Prime thought was "corrupted", but a survivor, friend and the original superhero. Killing him nothing more than blind and callous fury at him stripping him of his powers.
In one of the Infinite Crisis lead-ups, Villains United, Alexander Luthor (under the guise of the regular Lex) orders part of his Secret Society of Villains to retrieve a number of people for unknown reasons, chief amongst them the heroes Lady Quark and Pariah (who fought along side him during the original Crisis). Alex mocks and kills Pariah and later uses Lady Quark to power his dimensional tuning fork. Again, they were all heroes who tried to save the Multiverse together. And in the actual Infinite Crisis, he just kept on going with his despicable manipulations of everyone, including Golden Age Superman.
For starters, the kickstarter of the whole plot: Joker kidnaps Lois and links the trigger to a nuke set in Metropolis to her heartbeat. He then uses Kryptonite-laced Joker gas to make Superman think he's fighting Doomsday when in reality...it was Lois. The kicker? Lois was 'pregnant'. Not only that. Joker said he decided to go after Superman because it was "Easy mode" for him and he was tired of losing to Batman. Joker's actions were Superman's Start of Darkness.
And then we have Superman himself once he goes full-blown Knight Templar. His contenders include:
Throwing Atlantis in the middle of a desert after Aquaman defied him. Mind you, that's a WHOLE POPULATION he terrorized, as Shazam later points out.
Crippling a hero who idolized the Justice League because he wanted to maintain his people's right to protest against Superman's group's actions.
Killing Green Arrow after he accidentally hurt Mr.Kent due to an arrow fired at him that bounced off his body. And that's BEFORE the game's events, where he further crosses it.
In Irredeemable the very first thing we see formerCape the Plutonian do is incinerate a little girl's mother, baby brother and father in front of her eyes before, it's implied, murdering her off panel. That's not even the first atrocity he's committed; we learn that he's just destroyed a city and murdered a large portion of the population with in it. And horribly murdered more than a few of his old friends (of whom the father was one). He shows no signs of slowing down in later issues. Including the complete annihilation of Singapore.
In JL: A Justice League: Cry for Justice, Prometheus ripped off Red Arrow's arm, and then killed his 5 year old daughter Liane along with 90,000 others in Star City (the latter part was released the same week as Ultimate Red Skull throwing a baby out of a window).
While The Joker was (save for a stretch during The Fifties and The Sixties) always portrayed as a total psychopath, it is generally considered that he crossed the Moral Event Horizon in 1988 when, within a few months, he crippled Batgirl for life and beat the 2nd, still-teenaged Robin (Jason Todd) to death with a crowbar.
He's breaking out all the stops in Death Of The Family, but one particular stand out point is giving the hyenas, that he and Harley raised, rabies. These are pets that he and Harley personally raised since they were babies! All this to spite Harley for not waiting and praying for his return (never mind that he didn't leave any indication that he was going to) and for getting together with Deadshot.
In Kick-Ass, Red Mist and his father, mob boss Johnny G, manage to cross this at the same time. After going undercover as The Mole to help arrange an ambush for Kick-Ass, Hit Girl and Big Daddy, Red Mist has already proven himself to be a very despicable character. Johnny G hasn't done much except play the stereotypical mob boss so far, but we know he's supposed to be evil. Hit Girl tries to fight back and Johnny G orders his men to shoot her, an order which they gleefully oblige, sending her flying out a window. Red Mist then begins to rave about how awesome that was, both of them stop being mere villains and cross the line.
Also the revelation that Big Daddy isn't an ex-cop and dragged his daughter into the lifestyle after running away from her and lying to her about the fate of her mother.
In Kick-Ass 2 they basically pull out all the stops to make the villains seem unstomachably cruel, clearly in an attempt to eradicate any surrogate thrill the reader might get from their exploits. Don't-give-a-shit wanton violence is such a primal power-fantasy, they really wanted it to seem as disgusting as it would in real life. Most common criticism is that they went too far in this, parts of the book are just really unpleasant to read.
The League of Extraordinary Gentlemen's Griffen was implausibly popular with fans in the first volume, never mind he was introduced raping teenage girls and shown casually murdering an innocent policeman. To make sure we realise he's a very bad man in Vol 2, he sells Earth to the Martians in Vol 2 — and assaults Mina. It's the latter rather than anything else that results in his Karmic Death — by which point a reader can't feel any sympathy for him at all.
Likely because this is a common sexual fantasy, of a pedestal-residing woman being available to someone who sees their social standing as lower than dirt. Some scenes in Hollow Man also play off of this, though they end in killing instead as the serum has driven to recipients batshit crazy by then. Essentially the male equivalent of a bodice-ripper, as far as the reasonings behind this go. (for execution purposes, see guys who got off to the movie Disclosure)
Some might say he crossed it long ago, in The Mighty Thor Annual #14, where he kills a man named Theoric, pretends to be him to manipulate his betrothed, Sigyn, and marry her, taking advantage of Asgardian law, which has no annulment or divorce agreements, to keep being married to her.
Even Loki knows he's crossed a line. After he overwrites his child self, he briefly whispers "Damn me"...before immediately saying "Damn you all".
And now Young Avengers is muddling the issue. Evil Loki is still playing the role of Kid Loki, but the true Kid Loki personality is "haunting" Evil Loki like a conscience, and Evil Loki admits he might be Becoming the Mask over time. Got it?
As of Loki's final appearance in Young Avengers, it turns out that before he died, Evil Loki created a copy of himself that would assimilate Kid Loki. The copy, however, has a conscience, and is haunted by guilt over assimilating Kid Loki.
The title character of the Lucifer comics punted dogs as a hobby (naturally), establishing him quite firmly as an epic Deadpan Snarker and Sociopathic Hero that was as amusing and Badass as those tropes suggest. This continued all the way until the Basanos arc, where in a rather impressive twist the Basanos actually mortally injured him... only for Lucifer to reveal that he had manipulated Token Mini-Moe Elaine from the start and tricked her into dying in his place. He might have redeemed himself later on (bringing Elaine Back from the Dead helped) but the writer mentioned that he considered Lucifer's destruction of The Mansions of The Silence Lucifer's point of no return, destroying billions of souls because he was impatient.
In the Planet X storyline, Grant Morrison tried to do this with Magneto, whom he considered a "mad old terrorist twat." Not surprisingly, it didn't take, and was quickly retconned away. Morrison at least had the decency, however, to explain Magneto's Out of Character behavior: possession by JohnSublime.
In the Ultimate Universe Magneto crossed it for good when he brings about the events of Ultimatum and kills millions of innocent people, including numerous mutants. Just to make it clear to the audience that he's too far gone to be redeemed, he callously kills Multi Man when Multi Man tries to stand up to him. His actions are so atrocious that Cyclops snaps and breaks his no killing rule for the first and only time and blows his head off out of disgust.
Mongul II, the son of the original Mongul, kickstarted his career as his father's successor by punching his own sister's head off to eliminate any in-family competition. He's only gotten worse from there until he tried to take control of the Sinestro Corps in a coup and was imprisoned in the Central Power Battery by Sinestro himself for it.
Marvel rogue Mr. Hyde was always a violent, brutal thug of a villain, but his true MEH crossing would have to be when he beat the Avengers' butler Jarvis half to death while Jarvis was tied up and helpless. Hyde did this just to hurt Captain America, who could only watch as the horrible scene played out.
In Negation, Komptin captures Kaine on Komptin's homeworld. He goes to his mother's place and stashes Kaine there until Komptin's superiors arrive. His brother, however, is secretly part of an anti-Negation resistance force, and he helps free Kaine. When Komptin's superior finds out he demands that Komptin discipline his brother or it's his job. Komptin thinks for a moment, and then viciously murders both his brother and mother. Prior to this point in the story, Komptin, while clearly a bad guy, was portrayed with a small degree of sympathy: a hard-working but misfortunate underdog who wants nothing more than to be a successful leader within the Negation. With this act, that characterization changed, and Komptin was afterwards depicted as colder and crueler, living only for his job - and for revenge on Kaine especially.
In No Hero, two members of the "superhero" team Front Line bring down a passenger jet full of people to give the newest member of the team Josh a chance to earn some good PR after his horribly mutated face was accidentally revealed to the public. Front Line has apparently been pulling stunts like this for decades in order to maintain their control over the world. No wonder the governments of the world finally got fed up with their bullshit and sent their pet killer Josh to bring them down from the inside.
While the man who would eventually become the Saint of Killers from Preacher had already a staggering kill record to his name (among other heinous actions), he was nonetheless a decent human being (at least compared to most of his murderous peers), and for a time lived a life devoid of killing people. However, he finally crossed over when, in the course of brutally avenging the peaceful life that he lost, he cold-bloodedly killed an innocent for the first time in his life, damning himself to Hell in the process. What came afterwards (including the Ratwater genocide) was merely a formality, as it's arguable that he was too far gone by then. Even this is arguable since the series Big Bad, God, is still considered worse. In fact, the Saint of Killers gets a happy ending by killing God and taking his throne, finally able to rest.
Also, partially subverted for Cassidy, who slowly approaches the horizon through many of the later books, then seems to cross over forever when he shacks up with a devastated and drugged-out Tulip after Jesse's apparent death scene, lying to her and keeping her stoned to keep her dependent on him. Seemingly subverted when Jesse takes his hand in a redemptive gesture after their big fight scene, the seemingly played straight when Cassidy uses this hand-shake as an opening to sucker-punch Jesse and cripple him, then subverted one more time when we learn that Cassidy made a deal with God to betray Jesse this way, in exchange for a promise that Jesse would make it out alive and ok in the end.
From The Reign of the Supermen arc, Mongul and Cyborg Superman atomizing Coast City with a series of spammed atomic bombs, killing seven million people.
This led to Hal Jordan going down a moral event horizon of his own when he killed all the other Green Lanterns and became Parallax (though this was retconned later).
Doctor Destiny's Diner of Death. JLA Villain John Dee may have kicked the dog when he shot a woman who had been nice to him, but the real proof that he had turned irrevocably evil came next issue. Sure, he would routinely plot to drive the whole world insane back in the silver age, but what he does to those six people over a 24-hour period will keep you awake for days. And the fact that he let the rest of the world tumble into madness at the same time shows that he went for both the macroscopic and microscopic level of sadism. Arguably the cruelest thing he did to those six people was to briefly give them back their minds in the middle of his sick game. When one of his victims demanded to know why he was torturing them, this was his response:
John Dee:Because I can.
This is useful in showing Dream's moral compass: he doesn't care that much, just wanting his stuff back and returning Dee to Arkham. It's also evidence of how the Sandman is not your typical comic. Dream doesn't have a heroic spaz attack, he doesn't beat up Dee, he doesn't even punish him. When it's all over, he gives Dee back the power to sleep. Dee once again becomes an old, addled man in an asylum. He may not have been redeemed, but he's not a Complete Monster either, even though even your most depraved Complete Monsters seldom hold a candle to what he did.
Either The Sentry or Void, as we don't know for sure which side of his mind was in charge, crossed the MEH when he ripped Ares, God Of War in half — suddenly all discussions over the Internet about if he should be a member of Avengers after the end of Dark Reign were replaced with discussions about how he should be killed or why having him anywhere near Avengers is a bad joke.
Drago Wolf is hated by nearly every fan of the comic, and with good reason; turning traitor and joining Robotnik? Not nearly as bad as browbeating your abused girlfriend into killing the leader of the resistance and framing the titular character for it.
To make her an even more sympathetic character, she had no idea what she was doing when she nearly killed Sally. Drago gave her a Sonic mask (to make Sonic look like the hero) that had circuitry in the eyes that turned everyone in view into a duplicate of Snively, Robotnik's right hand. So, to recap: he bullied her into committing murder (even of a villain), gave her a mask that made her confuse her leader with her supposed target, and causes Sonic to be framed on the side. Yeah, Drago's a bastard.
Robotnik himself—or rather "Eggman." He'd been spending most of his time since his return trying to re-establish himself as a credible arch-villain. Then along comes his Egg Grapes—a set of devices that drain the life force of Mobians to power his machinery. The process is fatal. Eggman re-killed off the echidna race with 'em. Charmy Bee was in one for less than a minute and his brain is still a bit scrambled. When the Grapes were introduced, his sidekick/robotic daughter asked why he would do this when there were many methods of power production that were much more effective. His response was that none of the other methods were "as fun."
The same goes for the original Robotnik, the man who once called himself Julian Kintobor. The man attempts to take over an Overlander city and ends up being rescued by Mobians. His response? Initiate a coup, roboticize countless Mobians and hold the planet in a death grip for ten years. Before his death at the hands of Sonic himself, he crosses it one last time by setting Sonic up for the death of Sally and proceeds to wipe Knothole off the face of the map (both get better)
Geoffery St. John. Grief-stricken by the death of his father, he learns of the trapped Ixis Naugus and starts conspiring in order to get revenge on those who lead to his father's death. When the rescued King Acorn asked St. John to recruit people for his Secret Service, he purposely recruited people with less-than-friendly reputations for the sole purpose of pinning the blame on them should his Batman Gambit fall apart and he be revealed. In short order, he double-crosses Sonic, gives Naugus one of the Chaos Emeralds, and helps initiate a coup that would indirectly lead to the destruction of the Freedom Fighters.
Thrash the Devil. The last of the normal Mobian-type Tasmanian Devils as all of the others have been killed off by time and are replaced by the mindless Devil Dogs they are today because of scientific experiments performed by the Echidnas. He enacts probably one of the worst cases of Disproportionate Retribution by exiling all echidnas except for Knuckles and Dr. Finitevus into another dimension, despite all of them, including Knuckles himself, having nothing to do with what happened to them. What's worse, Charmy's wife, Saffron, is caught in the crossfire and tossed in, too! It's made very clear to the reader throughout that Thrash doesn't care who he has to hurt or murder to do it, as long as he gets what he wants.
(In Reality Thrash exiling Saffron and Julie-Su was more due to legal issue than story.) Somewhat downplayed that he does care for other people as long as their not echidnas unfortunately and Thrash (as well as Geoffrey) Moral Even Horizon was caused by being Well Intentioned Extremist rather than being evil.
Metal Sonic was a non-sentient robot for most of the series but was finally given sentience in Sonic Universe #50. This very first issue of him having a personality has apparently murder Shard one of the most heroic and likable characters in the comic. This is only after a fight that Metal Sonic started by threatening to murder a comatose hospital patient in his bed. It should be noted that Metal Sonic's only complaint about what he did in that issue was that he wasn't getting to kill Sonic.
Doctor Octopus and the rest of the Sinister Six, Spider-Man's recurring Villain Team-Up, have all murdered a few times over the decades, but it usually happened to AssholeVictims, so most readers could shrug it off and continue to at least partially root for them. Then came the early 90's storyline, Revenge Of The Sinister Six (not to be confused with the novel of the same name). As part of their plan to take over the world, they invade another dimension and steal the highly-advanced weaponry they find there. And to test it out, they kill over 143,000 of that dimension's natives. Spidey himself is stunned, noting that the Six rarely if ever kill. Their bloody campaign continues when they return to Earth, killing at the bare minimum four dozen people (an explicitly given body count), and Octopus threatens to blow-up the world with orbiting weapon satellites if the assembled heroes don't stop fighting him. By the time it's over, a clearly soul-weary Spidey notes that way too many people are heading to the hospital or morgue.
An interesting example for another Spider-Man villain, Kraven the Hunter. Kraven claimed to have a code of honor, and didn't (usually) pursue enemies he believed to be beneath his prowess. When he killed himself at the end of "Kraven's Last Hunt", he seemed to validate his claims of being an honorable (if unhinged) man. Years later, when his previously unknown family starts showing up, we get a different image of Kraven, particularly after they resurrect him in "Grim Hunt". Specifically, he kills his wife, and was implied to have abused her in the past. The worst, though, has to be what he did to his remaining children, Alyosha and Ana. He pits them against each other in a battle to the death, and it's revealed that Ana won. Although Ana initially seems fine with this, when she gets into a rematch with the Scarlet Spider, she's shown to have a death wish from the guilt she carries of murdering her older brother. Father of the Year, Kraven certainly isn't.
Kraven still looks like a saint compared to his family. He never asked to be resurrected, and in fact that resurrection entailed a whole slew of murders, including Madame Web, Mattie Franklin, Kaine, the wife of the original Rhino, a mentally ill man who wanted to become the Rhino, and Bill Connors, the son of the Lizard.
The Lizard crossed the Moral Event Horizon when it ate Billy Connors, Curt Connors's son. This sent Curt's personality flying across the Despair Event Horizon into outright oblivion, leaving only the Lizard personality intact even when he was restored to human form. Even the Lizard seems to realize that it crossed the line and is plagued with guilt over Billy's death.
Connors appeared to cross the line himself in an earlier story, when he revealed that he'd always been in charge of the Lizard's actions (which included killing a lot of people). However this story also had him act like he knew that Peter Parker was Spider-man, something he'd never known, and was so poorly received that it's been ignored by every subsequent author.
Also, his MEH crosses with another : the below-mentioned Kravens manipulated Lizard into killing Billy, as part of a larger, sicker scheme to corrupt Spider-Man.
Spider-Man's foe Mysterio was an egotistical, second tier villain with Large Ham tendencies. Then came Kevin Smith's Daredevil arc "Guardian Devil", where Mysterio tries to drive Daredevil to madness in a scheme that involved the murders of an innocent teenage girl, her parents, a prostitute, and (indirectly), DD's girlfriend Karen Page. To top it all off, Mysterio planned to push Daredevil over the MEH himself by getting him to kill a baby Mysterio had set up as The Antichrist. At the end of the arc, Mysterio commits suicide, so we don't know if this new borderline Complete Monster status would have stuck. To implicate the baby as the Anti-Christ, he impregnated the aforementioned virgin teenage girl while she was doped up to see angels. Fortunately the exact method was not specified. And his motive for all of this? He was dying of cancer at the time, and wanted to die fighting a superhero. At the time he came up with the scheme his real nemesis Spiderman was AWOL and he didn't think fighting a clone was satisfactory. So he picked Daredevil instead because Daredevil had stopped one of his insurance fraud schemes in the past. His real goal in the end was to get Daredevil to kill him and give him the death he wanted.
An exquisitely cruel one in the Old Man Logan non-canon miniseries: he tricks Wolvie into seeing all the other X-Men as an invading army of supervillains. Wolvie kills them all. You could argue the real MEH crossing is the illusion being lifted in time for him to see the dying Jubilee saying, "Why?"
This will come as a surprise to some people, but SpongeBob SquarePants and his friend Patrick qualify in one of their Nick Magazine comics. In it, after they discover a gigantic bowl of ice cream, they find themselves in another world with people made of ice-cream related food that look just like the duo and their friendsback in Cikini Bottom. The dessert people welcome the two in open arms, which the two greatly return. And by greatly return we mean GO BATSHIT CRAZY AND EAT EVERYONE!! We don't actually see it, but we do see Spongebob and Patrick's psychotic faces as they attack...and the very next panel is them, incredibly fat, and the whole town of ice cream reduced to a lifeless desert. JUST. WOW.
In the Star Wars Expanded Universe, Raana Tey from the Knights of the Old Republic Series murders her own student, frames the loveable protaganist Zayne Carrick, hires mercenaries to spy on his family, and manipulates his ex-girlfriend into trying to kill him.
From the same series, Haazen crosses the horizon when we learn exactly why he warped the Covenant into knights templar, orchestrated the deaths of hundreds of people, and ruined Zayne's life- purely out of jealousy for Barrison Draay, who he already killed years ago. That Start of Darkness made Haazen a pathetic, strangely tragic figure - but it also prevented anyone from sympathizing with the present Haazen again.
Issues 45 and 46 are this for Chantique. Before it was obvious that she wasn't exactly the most stable individual, but these issues showed just how depraved she really is. She mindrapes Zayne by showing him 1000 years of pain and suffering, forces him to fight the slave he befriended, mocks him when the slave commits suicide, and she keeps him alive just so that he can drive Jarael away. And it works, with Zayne failing to realize that he was deceived until it was too late. And she does it all to get revenge on Jarael.
Also, the little surprise she prepared for her father Demagol. Just these words:
Demagol: "Where are the children? You said they were out here."
Jarael:"You didn't pay attention. I said they were IN the courtyard"
In Star Wars: Legacy, Darth Krayt, in a fit of temper after losing an important shipyard, orders ten percent of the Mon Calamari species executed on the spot and the rest imprisoned in camps to be worked to death. Even the other Sith thought it was a bit much. As for the reader...
Transformers: Last Stand of the Wreckers: One can pick and choose when Overlord crossed it. There are three big standouts: his earliest actions which were murdering the Decepticon commanding officer, storming the prison, and tossing the Warden to the inmates. When he casually murders Rotorstorm by blowing his cranium open, or at the very end, when in the climactic fight, he tells Springer that all the guards and Autobots they were trying to save he ordered killed as soon as they landed.
Ultimate Red Skull was shown to be a typical evil bad guy, and then he threw a baby out the window. Immediately afterward it's heavily implied he had his henchmen rape the mother.
Immediately before this he had told the mother that he'd let the baby live if she would be so kind as to kill her own husband with a pair of dull scissors.
In Uncanny X-Force, Archangel, corrupted by the "Death Seed" Apocalypse placed in him way back in the original run of X-Factor, crosses when he kills Autumn Rolfson, a frail, likely sexually-abused anorexic woman, and one of his own followers, for objecting to him raising her son, a radiation-powered mutant, to be a mass-murderer. This somehow ends up having more gravitas than when he depopulated a town 2 issues earlier.
Vandal Savage, being an immortal villain in DC Comics continuity, has had several millennia and hundreds, if not possibly thousands, of opportunities to cross it, with just a few of the examples we know about listed on his character page. Of those listed, however, perhaps his most disturbing MEH-crossing candidate is setting up his daughter Scandal to be raped because he wants an heir.
The Governor from The Walking Dead explicitly steps across a new one of these (or is revealed/implied to have done so) in any scene where he appears for more than a page. By the time he is eventually killed by his own people assaulting the prison after making them execute about 90% of the cast including Rick's wife Lori and their infant daughter Judy, his MEH crossings look like the line of doors from the opening sequence of Get Smart.
Curious example from Watchmen, in that many readers think Big BadOzymandias's crossing of the Horizon and the greatest crime he commits are two distinct things. Sure he depopulated New York, but he had a damn good reason for doing that (preventing the Cold War from going hot and depopulating the entire planet). But when he gives a dozen innocent people cancer to discredit Dr. Manhattan, cold-bloodedly murders his absolutely loyal refugee servants to prevent them being a loose end, and pulls a You Have Outlived Your Usefulness on his loving pet Bubastis in an attempt to kill Dr. Manhattan, which turns out not to work, it becomes a whole lot harder to sympathize with him.
The Before Watchmen: Minutemen mini-series adds another potential crossing point for the Comedian - He framed Hooded Justices for the crimes committed by Rolf Muller by kidnapping some poor kid from Nite Owl's neighborhood, which led to Nite Owl accidentally killing Justice. The worst part? It's implied that Hooded Justice was actually one of Muller's victims.
Sabretooth is well-known in the MU for his hideous cruelty and boundless sadism and has done far, far too many awful things in his career to list, but his worst example involved his befriending Daken and playing the role of the father figure that Daken so desperately yearned for, all the while slowly grooming him for a confrontation with Wolverine. That confrontation did indeed occur, and Wolverine was forced to kill his own son as a result. Why did Creed do this, again? Simple: he just wanted to hurt Logan on the absolute deepest level possible, and forcing Logan to kill his own son accomplished just that. Oh, and it probably goes without saying, but Creed feigned every single bit of affection and saw Daken as nothing more than a means with which to hurt Logan, and he made it clear that he would have unceremoniously murdered Daken himself had he sensed that his manipulations weren't taking.
Miek of the Warbound crossed it during World War Hulk when he stabbed Rick Jones. Also, he is the one responsible for the destruction of Sakaar (he didn't cause the explosion, but he knew it would happen and said nothing) so that the Hulk would go back to being the Worldbreaker.
If the X-Men villain Vulcan didn't cross the Horizon when he killed Banshee as well as a lot of other people, then he definitely crossed it when he murdered his own father Corsair..
Some time during the X-Men Messiah Complex crossover for Bishop. The major incidents are recounted here allowing you, the reader, to draw your own conclusions. It could be that the whole crossover event is one giant Moral Event Horizon for Bishop.
He waited until the other adult X-Men were away from the mansion pursuing the new mutant baby, the Marauders, and the Purifiers. He then stole some of Cassandra Nova's nano-sentinels and had them take over the O.N.E. Sentinels and set them against the mansion. The kids had to defend themselves against an attempted genocide. Bishop blamed that on Cable.
Then he, in secret, made it to Dallas, attacked Forge, stole time travel tech, attacked Cable, and blamed Cable for the attack. The attack on Cable is directly responsible for the Marauders getting baby Hope. The panels imply that had the Marauders not attacked, he would have killed baby Hope right then and there.
Bishop also lied to the X-Men and blamed Cable for his actions, and tried to murder Cable and the baby again and again on Muir Island. His last attack (just after Cable and Hope time-slid away) nearly killed Professor Xavier. And in the subsequent Cable and X-Force titles, He Got Worse.
Zsasz using kidnapped runaways and orphans for gladiator matches and dumping their bodies in the river. You know it's serious when the current Robin is vomiting over what happened. The "Current Robin" is Damian Wayne. A character who, in his first appearance, decapitated two of Batman's rogues, assaulted Alfred, and tried to kill Tim Drake at least three or four times. And while he had mellowed out by that point, he was still pretty unflappable.
Lex Luthor giving a few thousand people superpowers in 52, then taking them away again. While they were in mid-flight. Because he was pissed the power-giving treatment wouldn't work for him. Luthor's a sociopath, sure, but he's not usually that petty (okay, there was that one time he stole forty cakes...).
The real reason is actually far worse. He thought Supernova was Superman in disguise (he was actually Booster Gold) and so created the situation just to test him, reasoning that Superman would use his powers to save the people. So in other words he killed thousands to test a HUNCH. And no, it didn't work — while Supernova saved as many people as he could, it didn't help Lex or anyone else closer to figuring out Supernova's ID.
Judge Death decides that, as all crime is committed by the living, everyone must be sentenced to death, and uses a mixture of psychic and nuclear weaponry to wipe out all life on his planet.