You can scroll down this page to find out what he did. If you want to, that is.
[A] person is almost always forgivable...'Almost' being the key word here. But you have passed that point of no return now, and I will never forgive you for what you've done here tonight. Trying to kill an innocent person out of greed... You are incapable of redemption.
In Amazing Agent Jennifer, Jennifer's parents definitely have crossed it in the "Coming Home" arc. Jennifer chooses a career path that is not the family business, and her father Benjiro turns his back on her, saying she turned her back on the family and the family business, saying Jennifer broke her mother's heart when she didn't come home for the holidays 3 years before the arc, at which point his Fantasy-Forbidding Father attitude had already driven her away, and refused to show up for her graduation, and actually forbid his wife to attend to the ceremony, and when he finds out Jennifer's mother did attend to the ceremony, she tries to pretend she wasn't there, and after shit hits the fan, Jennifer's mother accused her of "tearing the family apart". And in Jennifer's words, they wouldn't even try to make things right or listen to her. And as the final kick in the dog's gut, Benjiro outright calls Jennifer a disgrace. Yeah, she's a disgrace for not being their puppet robot, good at running the "family business" and choosing to live her life on her own terms instead. Parents of the year... And years later, in the series that Jennifer is a prequel to, they try to sweep all of this under the rug as if nothing had happened.
In Captain SNES: The Game Masta, Mother Brain crosses this when she shoves Duke into the Omega Gate, thereby introducing permanent death into a world that had extra lives, continues, and save points.
Joel Calley of Concession definitely crosses the Moral Event Horizon during the various acts of his complicated revenge plot. Among the worst are causing a child's parents to die in a car crash and giving his friend Artie cancer in order to kill him.
Artie himself had a moment, making the oil tanker crash into the Dolphin city. However, it's ambiguous if he actually fully intended to do it.
It's debated on the forums, but Syphile from Drowtales started as a Jerkass with slight hints of Jerk with a Heart of Gold, but then she left Ariel in a room with her pet cat for a week with no provisions for the calls of nature, resulting in Fuzzy becoming very sick with Ariel unable to do anything about it. When Syphile came back and saw what had happened, she blamed Ariel and took that cat away, and then when the cat bit her, she slammed his head against the wall and killed him. To top it off, when Ariel was understandably upset about this, Syphile beat her to make her stop crying. Earlier versions of the story had portrayed Syphile somewhat sympathetically (possibly unintentionally), but this act cemented her as a total bitch in many people's minds.
Quain'tana crossed it for many people at the realization that she had ordered her own daughter raped in an attempt to conceive an heir and then stole the baby Mel managed to have with Zhor, on top of the many, manyhorrible things she's done to her other children, but those particular moments seemed particularly callous and cruel compared to the others.
And now we have this. Now, we, as the audience, know that her paranoia and distrust of Mel is actually not unjustified, since she's in bed with the enemy, but breaking your own daughter's arms? That's way too much. Even worse in that she originally wanted Lulianne, one of the few people Mel can call a friend, to do it, but later did the deed herself. At least Syphile attacked her with the express intent to kill, making her death more justifiable.
And then related to the above, Shinae's baby is born horribly mutated and (implied) already dead as the result of said faulty seeds. Snadhya'rune considers sacrificing her own family, including unborn children, perfectly fine.
One of the remarkable things about the writing in Errant Story is that many readers were still undecided about whether Ian's actions were evil when he started trying to genocide the entire Elven race, due to what they'd done to the Half-Elves. Others thought the genocide was definitely evil, but were sympathetic to his motives and wondered if he could be brought back to his senses. Ultimately, it wasn't so much any one specific bad thing Ian did that showed he was beyond recovery, but this rant as his plans unraveled.
In story, many of the people who know Ian consider his attempted Suicide by Cop that destroyed a city his Moral Event Horizon.
Even after he'd pulled a heinous stunt like utterly humiliating Ally in front of the cancer patients she was a nurse for, Keith in Fans! still had some Draco in Leather Pants status from the people who believed he was still just on his revenge kick for the mental torture Ally had put him through years ago. Then he murdered Rob, who was both in love with him and was his most fervent supporter, in cold blood for the sake of power. Any and all sympathy vanished, and the forum unanimously cheered for an asskicking at the least.
Said cheering grew all the louder when he followed it up by setting a kid with cancer on fire just for talking back to him.
Fuzzy Knights rather plays with this a bit. Hamaestra has been working on his plans for years, but thus far had remained at least a Technical Pacifist. He realizes that killing The Fuzz will be crossing the line, and spends a strip getting completely drunk as he contemplates whether to do it or not, despite seeming completely confident when confronting his enemies later. The author's commentary included his thoughts that a lot of villains probably have times like that.
In General Protection Fault, Ki stood by her fiance Sam, even as almost certainly true rumors of him cheating on her spread. When she decided to have sex with him, only to be unable to go through with it, he tried to rape her in a fit of rage. When he came to apologize, she told him that she would not be able to forgive him for that, and returned his engagement ring, breaking up with him in the process.
Debatably, Zola in Girl Genius had already crossed this by double-dealing and out-Eviling the Other, but after this it's not really a question of if she is going to die, but whether death's enough.
Dellyn Goblinslayer from Goblins is a horrible bastard who tortures so-called "monstrous races" to get a better understanding of their biology and kill them more efficiently (at least, that's what he says, but he's a horribly sadistic monster), so he's well past that point when we meet him. However, he becomes this in the eyes of Minmax (who doesn't know about the rest of his horrific actions) when he admits to healing his "pet" Yuan-Ti after beating and raping her into negative HP. He ponders over what Dellyn says for a moment, then chucks the bastard through a window.
Kore is so far past the Moral Event Horizon in his FIRST APPEARANCE. He slaughters a bar of completely innocent orcs and what not simply for the fact they're existing. Then kills a dwarf child, who was adopted by one of the orcs, simply because he was WITH the Orcs. That's nothing compared to what he doesto Chief...
Kore by murdering the Dwarf child in cold blood just for being kidnapped by the 'evil races' and thus being 'tainted', Dellyn by carving the word 'monster' in Fumbles' head with his knife during their first torture session. Psion-Minmax does this by telling Kin how he murdered her 817 times, and she remembers each of them thanks to the power of the Maze of Many, and how he intends to use the powers of the maze to wipe all all the characters currently inside completely from existence. Before killing her for the 818th time.
In Grim Tales from Down Below, Mandy masterminds 9/11, Operation Freedom, Hurricane Katrina and Crash 38 in an attempt to win Grim's affections..
Despite the controversy, when Bleedman held a poll over whether or not to take down the page showing this crossing of the MEH, fans voted to keep it up.
It's been shown that Grim caused most of the tragedies of history. Collecting the souls of the dead is one thing, but engineering tragedy and suffering is quite another. If he's responsible for the holocaust, that puts him far over the horizon as well. Though Grim is The Grim Reaper and most likely is required to do such things, possibly putting his actions under Blue and Orange Morality.
In Gunnerkrigg Court, we got an example that was more subtle than most, but still important, in the flashback to the Court's early history. Dogged Nice Guy / Stalker with a Crush Diego reacts to his vehement rejection by Jeanne by nominating her as the sacrifice for a plan to "protect" the Court from the magical influences of Gillitie Wood. Even after that, he still had groups of people who insisted it was just a misguided plan to play hero, that he really didn't mean it, or even that Jeanne deserved it, until the next page went up, wherein Diego smiles at an unknowing Jeanne's fear of being sent down into the ravine. After that, even the people who'd previously defended him wanted him dead.
Jack Noir of Homestuck has gone from Obstructive Bureaucrat right through One-Winged Angel. Listing all his atrocities would fill up a decent chunk of this page with spoiler text. Despite his murderous desires, though, Bec's prototyping prevents him from killing Jade personally.
Fans debate quite a bit as to whether Vriska has crossed this or not. Some viewers believe that she gleefully swan-dived over the line back when she mind-controlled Tavros into jumping off a cliff and has only gone deeper since then, and some viewers think that she can still be redeemed. Given that she's easily the most divisive character in the series, this argument is not surprising.
Eridan has the distinction of crossing the Moral Event Horizon AND the Despair Event Horizon at the same moment. Convinced that there's no hope left for the Trolls, he defects to join Jack Noir, killing Feferi, Kanaya, and his species' last chance for revival in the space of about two minutes. And in a good part, it's for a really petty reason: Feferi decided to not be his moirail anymore, due to being emotionally exhausted after having to keep him in check.
While Doc Scratch does work for the comic's Bigger Bad, Lord English, he always acted more Affably Evil. Then we saw how he treated Aradia's ancestor, the Handmaid. Nearly suffocating her, beating her with a broom, keeping her alive for the sole purpose of using her to make her home planet a Crapsack World, and finally, setting her up to be murdered by her successor. Yikes. By the last point, she had been asking for death for millennia, begging for her slavery to Lord English to end.
Caliborn gleefully killing his own split personality/sister Calliope's dream self, then laughing his ass off when the Alpha Kids are apparently slaughtered by Red Miles, make it pretty easy to believe that he is Lord English's past self.
Also, he mercilessly guns down the first living creature that he sees in his entire life, Gamzee, made worse when later flashbacks reveal that Gamzee raised both him and his sister from infancy.
Later, he seems to find intense pleasure in informing Jane of her father's supposed horrific, torturous death in the prisons of Derse, on her birthday no less. This didn't actually happen, but the thought of it is enough to drive her to tears.
Grimbark!Jade kicks WV off a ledge with full intent for him to die horribly in the lava below. He doesn't die and she is mind controlled at the time, but still...
Aranea drops a house on Jade to prevent her from going Grimbark a second time, then manipul8tes the God Tier clock into giving her a Just death just as it appears she might revive, killing her for real as a result.
"Housepets!" has Pete, a recently freed griffin with supernatural powers, who recently gave a former member of PETA a Karmic Transformation into a dog. This was seen as an interesting twist, and King, as he's now called, has become a fan favorite for being complicated, adorable, and the Grumpy Bear of the cast. Except that on the conclusion of the Chrismas ark, Pete informed King that he has no intention of changing King back into a human.Horror ensued.
Drip, embodiment of the Sin of Lust, likely hurled himself across it at some point in the backstory, as we know he was a murderer and rapist in life. Then he did...this. However, his first (chronologically speaking) crossing of the MEH would have to be when the Devil takes him back in time to murder his own parents. Not that he knows who they are, but given the sheer brutality in how he murders his father and handles his mother (WARNING, VERY NSFW), it almost doesn't matter. (Even he is horrified by this once he realises who they are!)
The Vorshes stuck a living, conscious woman in an oven, cooked her, and ate her the first night they met.
Veithel of Juathuur thinks she is past that because she deserted the Juathuur, getting her master blinded in the process, and making everyone believe she was dead.
Jamie's mother from Khaos Komix crosses it when she forces Jamie (who was at the time probably around 5/6) to wash himself in scalding hot water to cleanse himself for the crime of simple curiosity.
She officially crosses it when it turns out she knew Jamie was raped and did nothing to stop it.
Natalie Geln, whilst never nice (Her first appearance is her as a five-year-old bullying Charlie for wearing women's clothing), crosses it when she orders her gang to rape Charlie's FTM transexual friend Tom.
Xykon in The Order of the Stick zombified his grandmother and several other people, then had them kill his parents. After that, he was your basic stereotypical villain, doing evil things for no other reason than he can. He didn't really have a Moral Event Horizon, because he was a evil horror since birth who thinks morals or even a reason for what you do is for pussies. As for what he did on the top of the page, a group of paladins were standing against him, ready to fight to the death, and he threw a super bouncy ball with a Symbol of Insanity inscribed on it, causing them to go mad and slaughter each other. Only a few of them didn't go insane, on virtue of not seeing the ball, and the only one of them that survived was paralyzed by Xykon, so he would be Forced to Watch him zombify his dead friends. To give you the full quote of what Xykon said:
Xykon: I tell you, Ugly, nothing's funnier than false hope. You really thought you had a chance there for a second. I mean, sure, I could've just blasted you all from above with fire and lightning and such... but I've always felt that when it's really important, it's worth it to go that extra mile. Don't you agree?
And just to really punch the reader in the gut, one Sapphire Guard is left atop the carnage, screaming "My God, What Have I Done? Forgive me!", before committing suicide.
Redcloak himself isn't as bad as his boss Xykon, but he also crosses the horizon when he murders his brother Right-Eye simply so that he can fulfill his plans of The Dark One gaining control of the Snarl.
To say he killed Righteye "simply" to fulfill the plan might be a bit harsh. He had by that point invested the better part of his (for a goblin, substantial) lifespan into the plan note The plan being to blackmail the god into giving the goblins a fair deal and sacrificed too much to simply stop. Still a moral event horizon though.
Vaarsuvius while under the influence of the soul splice could count. On the one hand, it allowed them to save their family from a black dragon seeking revenge, but on the other hand, casting Familicide was arguably taking things a step too far, especially since it went as far as to kill every being related to the black dragon and every being related to those beings, which included not only Girard Draketooth's family, but all the innocents the Draketooths used to propagate their clan. To make things even worse, it's later stated that the soul splice didn't have an effect on V's alignment; V willfully committed mass murder of their own free will.Needless to say, V didn't take this fact well.
Since then, Vaarsuvius has taken part in torturing a prisoner. Without any such excuse as V had for the above.
In Scary Go Round, former Designated Hero Rachel Dukakis-Monteforte's psychopathic jealousy of universally popular heroine Shelley Winters finally drives her to arrange Shelley's death via biker gang. The author confirms her crossing of the M.E.H. by having her sell her soul to the devil and skip town. When she returns a few years later, it's only to have a bridge dropped on her.
Kathryn: Uuggh! Hmph. Well, the good news is that I can now start killing and not feel in the least bit guilty. The bad news is I'm not going to feel the least bit guilty about the killing I'm about to do.
Subverted, after a fashion, in Sluggy Freelance when Aylee has a Face-Heel Turn, and attacks the Main Characters' Halloween party. She doesn't actually kill anyone, but she makes it perfectly clear that she could, taunts Riff at how helpless he was at protecting anyone, and threatens to do it for real if he meddles in her affairs again. The subversion comes when we find out that this wasn't actually Aylee, but an evil clone instead.
Played straight by Dr. Schlock when he orders the murder of Feng in front of Oasis to try and get her to cooperate. And it can't be blamed on the Fate Spider either, as it is revealed by him that if Feng had not been manipulated into going there, then Schlock would have ordered Kusari to murder Ms. Zalia in front of her own daughter instead. He went from an affable, Ineffectual Sympathetic Villain (or possibly Anti-Villain) who was always on the run to the Big Bad of the strip's Myth Arc. Murdering Feng just sealed the deal that he can never go back.
In Mr Death's now finished Something!, largely a retelling of Mega Man X 4 and Mega Man X 5, Double, who had been established as a villain, but had mainly been the butt of jokes, ended up crossing the line in the eyes of the comic's readership when it was revealed he was the one who sent Iris to her Plotline Death. The incident changed the outlook, in general, from "I can't wait to see the fight with him!" to "Sonuvabitch needs to die."
Kyle in Something Positive, here. Bad enough that he's a misogynistic jerk, worse that he's cheating on a member of the cast, but what completely pushes him over the line is that he's bragging to a reforming Jerk Ass who would move heaven and earth to have the woman Kyle's discarding. His comeuppance delighted the bulk of the comic's notoriously Unpleasable Fanbase.
Autumn, the Stalker with a Crush who tries to kill the girl her crush is taking to the dance, for example.
Or, in the "Ty and Rosaline" arc, Ty not only abuses his loving wife repeatedly, but drives her to contact Suicide for Hire...and, after her suicide, makes the fatal mistake of contacting them himself to arrange his suicide... and demonstrates a complete lack of interest for his wife's last message. They do...oh, don't they just!
Admiral Blake in Terinu. Supervising Terinu's arrest and detention in a biological research lab? Decidedly unpleasant. Arranging for his daughter Leeza to be fired from her job and forcing her into being the boy's guardian after she rescues him from said facility? Definitely not good. Having Leeza arrested and ordering Terinu's assassination to cover up a centuries-old act of genocide? The readers were howling for his blood.