A:TLAR - Warden Liruk, of Chapter Four: Shooting one of his own guards in the head as a demonstration of how he uses his Weapon for Intimidation, so that Katara would willingly go to his private quarters for...let's try not to think about that.
The Enforcers, of Chapter Six: The brutal murder of Xin Fu on the grounds of gambling debts certainly counts.
Pick any moment from Jet's backstory section in Chapter Eight.
This will be updated as the series progresses.
In How I Became Yours, Mai killing Katara's unborn baby via poisoned fruit is meant to be this. For more objective readers, it's actually Katara bloodbending Mai to death (and in broad daylight), when she was perfectly capable of merely incapacitating her so Mai would be taken to trial for attempted murder.
Hungary may have crossed it for some readers, losing the sympathy points from the first incident when she teaches Liechtenstein to become Prussia's lover alongside her, so they can get benefits from it. Yes, a rape victim coaches another into giving sexual pleasureto their common rapist. Others may consider it more of a mere Kick the Dog brought by the harsh circumstances both of them are caught in, though.
iFight Crime With Victorious has Robbie abusing his power to make Cat give him a blowjob, Beck losing control of his power and burning Jade alive, who comes back to life later thanks to her power and tortures Beck in vengeance, Carly abandoning her friends due to obsession with having a super power and not sharing it with anyone, and Neville killing Sam in the first timeline to torture Carly. Many of Megan's visions also predict these events happening to various characters.
Marble crosses it when she poisons most of Ponyville's remaining foals. Though things get a bit more complicated when it turns out that she had actually refused to do so, so Actrisepossessed her in order to do it herself. Which action is worse is debatable, but what is clear is that it's Actrise's MEH, not Marble's.
Most of The Prayer Warriors cross it at one point or another note The series could be considered a crossover of Harry Potter and Percy Jackson.
Jerry crosses it when, after learning that his wife Mary is pregnant with the child of Percy Jackson, he orders her thrown off a high tower, stoned to death and beheaded. This doesn't stick, but it is proof of how far he will go to punish sinners.
Michael crosses it by carrying out the sentence on Mary.
Jason crosses it by putting napalm down Socrates' throat, then after proving that he's not a god, leaving him to have a slow death by burning alive.
Draco, for all his wearing leather pants, so to speak, crosses it by leading a mob with the intention of killing the entire British Royal Family, "men womenchildren and all". He succeeds in killing the Queen, and plans on hunting the rest down after they escape.
Arguably, the act that makes Dumbledore stop being a Designated Villain, at least for the moment, is having Noah executed by being stoned to death because he prayed to God and Satan. Notice that the Prayer Warriors approve of this.
JohnLennon crosses it by nailing a group of Christians to corpses and setting them on fire.
In-Universe, several vilains are considered to have crossed this by doing things that would come off to some people as completely harmless (Venus kissing another woman), or Pet the Dog moments (Stalin deciding to allow freedom of religion).
In the series Lex Luthor Triumphant Lex Luthor grants Lois Lane an interview 8 months after Superman vanished without a trace. He calmly tells her how he captured, lobotomized, skinned Superman; he also tells her it's an ongoing thing. Selling Superman's eyes, skin, teeth, and fingernails has made Luthor even richer than before. Which, of course, means Lois isn't making it out of his building alive.
In the Death Note fanfic Story Of The Century, Light and Misa cross over this towards the end. Believing that Rem has already wiped out everyone on the task force, Erin goes to confront Light and Misa, to find Light writing his father's name in what he believes to be Misa's notebook. The instant Erin tackles Light, Misa proceeds to give Erin a beatdown, sees her real name with her Shinigami Eyes, and shouts it to Light. She holds Erin down while Light writes Erin's name in the notebook. Although it's implied that Misa is beginning to doubt Light and their mission, she defends their cause fiercely and even kicks Erin as she begs them to realize what they're doing.Although Light tells Erin that he's sorry, he insists that he's only doing what's good for the new world, not seeming that sorry or concerned about Erin's thoughts on the issue in the least. It's assumed that they would have gone on to kill everyone else on the task force afterwards had they not been thwarted. From their viewpoint, it was probably justified, but still...
L's actions towards the end are treated as crossing over this, at least due to intention rather than the action in itself. After Erin overhears Light and Misa discuss their plans, she tells L about it in secret. Without the task force's knowledge, he has Wedy break into Misa's apartment to steal her notebook that she'd dug up, and replaces it with a counterfeit made by Watari. He has the deaths covered on the news channels to make it seem as though Misa's still killing criminals. Then, to give himself immunity to the Death Note, he writes his own name in Misa's notebook and gives himself 23 days. He uses this also to manipulate Soichiro and the task force into going along with the rest of his plan, which includes pretending to test the 13-day rule in the notebook using two death-row inmates and luring Misa back into a trap. Not knowing any of this, Erin tries to destroy the notebooks (despite one of the fake rules) and free Light and Misa from their influence, only to have L tackle and knock her out so she can't interfere. The reason this is considered his MEH is because, at least according to Erin, he was more concerned with exposing the two Kiras and solving the case rather than actually saving anyone. Letting her destroy the notebooks would be destroying the evidence, essentially letting Kira get away for good. He doesn't deny it when she calls him out on it, either...
The Firefly fic Forward introduces Inducer One-One-Nine, another Academy escapee who seems like she might be just as sympathetic as River....and then leaps across the MEH in the span of a single chapter, when she reveals she's been behind a violent murder spree and the massacre of a village, and then uses her Psychic Powers to force a nearby man to shoot and beat Mal into a coma, Mind Rapes River into being her protector/bodyguard, and kills the entire crew of a freighter as a distraction. And that's just the start of it, as in the next part of the story, she forces a man to commit suicide just because he worked for Blue Sun, and then causes a space station of over eight hundred people to go berserk and start killing each other just to delay a pair of Hands of Blue.
This doesn't make her death in any way satisfying for the reader, or the person who pulls the trigger. Inducer One-One-Nine is, at most, 12 years old. We also get to read a snippet from an leaked internal document from the Academy about another subject going through "combat testing", during which the report suggests that further trials should be suspended "until the subject enters adolescence"; the Mind Rape they were inflicting on River is also being tested on pre-teens. If the Academy didn't already cross the Moral Event Horizon with what they did to River, they sure have now.
When Elly goes insane and he refuses to seek treatment for her.
When he attacks Elly upon finding out she had an illegitimate daughter and point blank revealing Michael's not his biological son-something he did not know
When he brought his fiancee to his wife's funeral. It's worse in that said fiancee was Kortney Krelbutz, who had stolen from Elly's store and threatened to beat up April over it. Not to mention both the courtship and the engagement happened before Elly died.
For what it's worth though, what some characters view as his MEH was when they discovered that the still insane Elly had been mistreating and starving their elderly dog Edgar...and he thought it was hilarious.
Speaking of Elly, she retroactively crossed it when Elizabeth and Anthony figure out that she was behind Elizabeth's Attempted Rape to invoke the Rescue Romance trope on them.
In Boys Und Sensha-do, Miho's mother Shiho crosses it when she visits Miho in the hospital for the sole purpose of announcing that Miho has been disowned.
in Boys Do Tankary, several of Vincent's men cross it in a flashback when they rape and murder some teenage POWs. When Vincent refuses to do the same to Nyra, the remaining girl, their superior has him and Nyra locked up in a room with a gun for a month, and they are told that if one doesn't kill the other by then, they both will die. Vincent is forced to give in and kill Nyra but she turns out to have survived.
Cornelius Fudge in Never Alone Never Again. He sent Aurors to ambush the Wesley house to arrest Harry, attacking everyone including the unarmed ones. Then he ordered an adult dose for Harry, which could potentially kill him at his age and after Harry gave his truthful statement, Fudge didn't believe him and promptly tossed him in Azkaban. But one of the worst things he did was hide that Voldemort had returned and ordering his people to take the murdered victims and burn their bodies so no one would know about them. Dozens of witches, wizards and children were dead and more kept dying and all he cared about was keeping his image. His eventual fate seemed a bit too easy, considering what he had done, as Ron angrily pointed out.
In-universe, any pureblood who hears about it considers Voldemort creating a horcrux (let alone several) to be this in The Power He Knows Not Is. As far as they're concerned, the soul is the single most sacred thing in the world and creating a horcrux requires destroying your victim's in addition to mutilating your own.
Frontierstuck: The Homunculus Jack Noir has run through towns killing dozens if not hundreds of trolls and lusii. In chapter 13 he reveals himself as more than a mindless monster and punches through Kanaya fatally damaging her spine and killing her, telling Rose, his creator, that he's doing all this to make her suffer and he's having fun doing it, then he leaves her to contemplate the death.
In Queen of All Oni, the dark chi wizard Lung, Daolon Wong's former apprentice, crosses this when he captures and TORTURES Jade to try and break her to his will, and goes ahead with it even though it's clear she's DYING! If it wasn't for this (and his Undying Loyalty to Jade) Right having killed him would have been Right's, and Jade's feelings toward him would also qualify as being justified (she didn't order it, she was near death at the time, but she was glad he was dead).
Jade herself arguably crosses the line when she kidnaps Viper in order to transform and brainwash her into the new Samurai Khan General, Hebi. The reason it's arguable is that it's clear by this point that she's gone off the deep end as a result of the trauma from the above.
In Lilly Epilogue Family Matters, Lilly's father's first words to Hanako are insulting her burn scars, asking whether she got them by accidentally setting the stove on fire, and telling her that she will never find a good man if she continues "acting like a scared rabbit all the time," which causes her to have a panic attack. This is even worse than it sounds, especially considering that Hanako was scarred in the same house fire that claimed the lives of both her parents, only surviving because her mother shielded her from the flames, and because Mr. Satou was warned about Hanako's scars and sensitivity in advance.
In the Lyrical Nanoha fanfic Graceful Degradation, it is Precia acknowledging that her actions have a good possibility of destroying the Earth and going through with her plan anyway since she considers it an acceptable risk and sacrifice to revive her daughter.
In the Mass Effect fanfic The Council Era, those who've read the series consider it extremely debatable exactly when certain characters (especially the Villain Protagonist Tyrin Lieph and his Worthy Opponent antagonist Halak Marr) crossed the irredeemable line. Tyrin commits numerous horrific acts, including the mass-murder of the dezban race, the dissolution of the manaban people's religion and government, and the murder of 150,000 krogan in order to force their leader into a compromise, but he's also fully aware that he deserves and will receive consequences for his actions. However, he has no regrets, believing that Utopia Justifies the Means and that he is Necessarily Evil. In the opinion of the series' author, Tyrin's Moral Event Horizon is his taking advantage of both his son's death and the Council's and public's sympathy for said death to sway them into supporting the genocide of the dezban race. He antagonized the dezba by killing one of their high chieftains, resulting in the dezban revolt, and thus orchestrated the deaths of 7000 civilians and soldiers in the dezban revolt in order to make the public support the dezban genocide.
In-universe: To his own devoted soldiers, Tyrin's MEH is considered to be planning to betray his krogan allies in an effort to completely purge the galaxy of their race.
During the ending of Part I, Marr threatens to unleash the power of a fleet of krogan cruisers to destroy the entire Citadel, in retaliation for Tyrin's poisoning of a portion of the krogan military forces, if Tyrin does not allow for the krogans to receive a council seat and the status of being the majority race in the Citadel military.
In Ace Combat: The Equestrian War, the griffins are ordered by General Silverbeak to kidnap fillies and use them as hostages to demoralize ponies in the first phase of the war. But it is revealed in chapter 14 that it was an order of Red Cyclone. Then, his plancomes into the picture...
There is also Gilda torturing Medley and threatening to rip her wings off after the latter saves Rainbow Dash, who was being subjecting to a brutal No-Holds-Barred Beatdown while being immobilized by four other griffins. Not only did Gilda betrayed Dash's trust, she later breaks Medley's wings anyway!
In Families Olive Branch and Speedy Delivery are already unpleasant characters to begin with, the former waging an anti-Celestia campaign using smear tactics and the latter constantly abusing Scootaloo. Both of them cross the horizon, however, when Speedy Delivery drugs and kidnaps Soarin and Rainbow Dash on the orders of Olive Branch, who also has Twilight assaulted and kidnapped as well. That is, if Olive Branch didn't already cross it when he ordered the dragons from Dragon Quest to attack Ponyville to make Celestia look bad.
The God Empress Of Ponykind: The Siege (and Sack) of Manehattan is one for Nightmare Moon and her Traitor Legions; The Daughters of Manehattan Legion is massacred (only fifty survive out of a thousand Legionnaires), any civilians that get caught up in the fighting are tortured and killed (including foals and an unborn child), and the entire city is burned to the ground.
It Takes a Village this is done in Chapter 10 by those who want Spike out of Ponyville when someone launches a harpoon at Spike while he's flying with ponies on his back.
In the MLP fanfic My Sweetie, during a flashback, Derpy's mom crosses it when she abandons Derpy in the Everfree Forest simply because Derpy's presence cuts into time shared with Derpy's dad. Sufficed to say, this troper was glad that Derpy's dad divorced her.
The Pony POV Series version of Discord did this, as far as in fic canon goes, by breaking Trixie so severely that she remained "Discorded" weeks after his downfall and even Twilight's memory spell alone couldn't free her (of course, due to the author not revealing Loneliness' origins, she may have been there even before Discord got his claws in her, but we can't be sure). Unlike the Mane Cast, Trixie wasn't a direct threat to him (yet), she was just similar enough to Twilight he could "warm up" on her first.
Discord crossed it even before we find out what he did to Rainbow Dash: He tells the whole bucking city of Cloudsdale that Rainbow Dash saved them at the cost of all else, and then drives them mad despite Dashie's pleading, and hammers it into Dashie's head that this is her fault. Then a couple chapters later, Applejack sees some of the truly sadistic things he did. For example, he made Derpy hate Dinky's guts and scream this newly-introduced hate at Dinky—at the time of the chapter, they've only recently stopped crying themselves to sleep, he turned the spa twins into an equine pushmi-pullyu—they still can't stand being near each other, and he turned Apple Bloom into one of the nightmares from Story of the Blanks.
Loneliness crosses this in her first appearance by impaling Twilight and breaking Trixie's leg. While she'd done some awful things before then, this was the point where it showed Trixie wasn't as much of a Morality Pet for her as it seemed.
In the Dark World timeline, we have the pure evil serial killer known as epilogue Fluttercruel, a horifying monster who kills every living thing she sees just out of sadisim. The line is finaly crossed when she reveals that she spent 500 years torturing Fluttershy til she was rescued by Cadence's spirit, made worse by the fact that she considers Fluttershy her mother and yet calls her her "favorite toy", her only regret being that she lost her and could not hurt her anymore. Though it turns out later she had a warped sense of logic and thought she was expressing her love to her mother.
Don't forget The Nameless Passenger aka Nightmare Paradox/Eclipse aka Twilight Sparkle, who became a Nightmare just to punish Discord for his crimes... then decided he hadn't been punished enough, so she trapped everyone in a time loop, just to keep punishing Discord, and not giving a flying buck about everyone else. In other words, she hasn't crossed the Moral Event Horizon, so much as pole vaulted it with a rocket-powered pogo stick.
Rainbow Double Dashs Lunaverse has a bunch of these. For some, the protagonist Trixie herself crosses one in the season finale 'At the Grand Galloping Gala' when she decides to commit highly illegal acts to defeat the nobles, because... they were commiting illegal acts to defeat her. Between the hypocrisy and the sheer magnitude of what she plans to do, some people think she's gone into villain territory herself.
To be fair, they're only highly illegal if she doesn't get the Princess' permission. She does, which transforms what would otherwise be treason into a convoluted police sting, which is questionable but not illegal
Her plan also ended up being a police sting to get a known criminal arrested, and not what was originally suspected, which was to poison the entire Night Court. Unfortunately, the revelation of this came too late for many readers
In It's a Wonderfully Splendid Magnificent Life, Tamaki's grandmother crosses it by threatening to withhold funding for Tamaki's mother's health care if he goes to see her while she is in danger of dying. Neither Tamaki nor his father have any reason to doubt her; Tamaki is pushed far enough over the Despair Event Horizon to wonder if he should never have left France, and Tamaki's father shows his contempt for her by not referring to her by anything other than "she", as "He did not want to admit to having any sort of the relationship with the woman who was, by an accident of fate and genetics, his own mother."
In the fanfic Brave New World, Dirty Coward Paul crosses this by betraying his clan simply because they locked him away after he stole a forbidden scroll, which leads to almost all of them being killed...with the exception of Dawn and Palmer.
Lord Zedd crosses the Moral Event Horizon in the infamous Agony In Pink by creating Tortura, a monster made from books about the human anatomy and torture, to torture Kimberly to death in order to demoralize the rest of the Power Rangers.
The first we're made aware of is Shredder (yes, he's in this; Continuity Porn, remember?) reorganizing the Foot by having any member with any standards whatsoever brutally killed, then dumping their corpses in the sewer for the protagonists to find.
The most extreme example probably goes to President Fullenton, who, while he had begun flirting with Knight Templar-ism while fighting the war against the revived United Alliance of Evil, initially seemed like a decent enough guy. Then he spectacularly toboggans down the slippery slope to become a President Evil by firing a Kill Sat at the nation of Sudan (which had allied with the UAE), burning every living thing within said nation to a horrible, horrible death. The later revelation that he's actually one of the leaders of the UAE, and had been playing both sides of the war to increase his own power and subvert the governments of Earth from within (the author's favorite villain in anything is Darth Sidious, can't you tell?) just seems like a formality after this.
Senator John Thorne, leader of an anti-alien hate group, crosses when he has his men massacre a defenseless alien refugee camp, treating us to scenes of soldiers doing things like burning down houses, then throwing alien children into the fire. Kat Manx, who was serving as a doctor to the camp, is the only survivor.
Mr. Stonefeld, crooked Texas oilman and Omniscient Council of Vagueness member, crosses when, after the Rangers leave for space to gather a resistance movement against Fullenton's aforementioned subverted government, he has a hit put out on their families still on Earth, to break their spirit. These are actual characters we'd recognize from the TV show, and we get to watch (err, read) them die in loving detail.
Biotechnology mogul Jacob Russell crossed retroactively, when it's revealed that he had Trini's lovable old Uncle Howard (again, an actual character from the show) murdered and his death made to look like a suicide for his scientific research.
Another retroactive crossing goes to Gregory Roschin, the head of the aforementioned Omniscient Council of Vagueness, when in the '80s he had the entire Yeti race wiped out for their fur (ever wonder why Norg was the only one we saw?). Stonefeld helped with this one, too.
And yet another retroactive one goes to Captain Logan (yes, that's right, the Big Good from Time Force), when it's revealed he shot Ransik's wife, simply for giving birth to a mutant (she was completely human).
In the spinoff Pretty Cure Heavy Metal, Ramirez uses an Akumana to possess W. Benny Bara, and Kuroimetaru offs Gacy without waiting for Lady Marilyn's orders. Even Lady Marilyn herself saw the act as the point of no return, warning him that she'd execute him if he wasn't so vital to Mansonia's security. It's no surprise, given his constantly killing innocent bystanders — he tap-dances over the Moral Event Horizon four times each day before breakfast. An example would be when he fed someone to the sharks and left a note reading "He disagreed with something that ate him." in episode 41.
Kainatrol from Futari Wa Pretty Cure Blue Moonerases her own sister from existence in front of the man she was going to run away with, then gives him the choice of joining the Etherium or being erased with the rest of the world. He reluctantly joins, hating himself for his weakness from then on — and she continues to play with his emotions and remind him of his failures, knowing full well what's going through his mind. For 150 years. No wonder they hate each other.
In the Sonic the Hedgehog fanfic Prison Island Break Head Warden Mephiles is consistently portrayed as pretty awful, but the specific moment you're supposed to realize he's worse than any of the cons is when he bullies prison doctor Amy Rose into performing humiliating sexual acts for his pleasure, while taking photographs to use against her as blackmail material in the future, just in case. And then just as he's about to leave, he turns around and tells her she is the one who is a disgrace.
In the Sonny With A Chance fanfic Channy's Struggle the books Big Bad Kyle Smith, rapes Sonny when her father see's favor in Chad. When Chad finds out and confronts him about it, Kyle simply brags that he was her first and she was amazing.
Yuyuko's Yukkuri Farm, one of DOS's yukkuri comics for the Touhou series, focuses on a Marisa yukkuri being cared for by Yuyuko, who at first seems to be quite caring to her, feeding her well and giving her friends to play with in the form of other yukkuris. The Marisa yukkuri loves Yuyuko, and as she grows, she soon becomes pregnant, and then gives birth to a koyukkuri (a yukkuri baby). Yuyuko picks the little yukkuri up before then proceeding to snarf it down right in front of its poor mother — and then telling the distraught mother yukkuri that the baby was nothing but a snack to her, and that this is the reason she's been caring for her. It just gets worse for the poor Marisa yukkuri from there, and by the time she's rescued by Reimu, she shows signs of acute PTSD from her horrible ordeal at Yuyuko's hands.
In Diary of a Psychopath, Urotsuki's already been proven to be an Axe Crazy serial killer. However in Chapter 7, after killing Kamen, she managed to get crazier. She becomes interested in Aoshiru, coming into him rather creepily and sexually. This culminates in Chapter 8, where she ends up kidnapping and raping him, keeping him locked up in an abandoned hospital room. She has no problems with this, and actively continues to beat and rape him until he becomes a hollow shell to brainwash.
From Yukari Is Free, we have Fedt-Geszhine, the bespectacled, diminutiveDragon to the planet-conquering AlienWarlord Gathuum-Sythaal. Fedt-Geszhine leapt merrily across the horizon at a young age, when he killed his mother so he wouldn't get in trouble for drilling a hole in his sister's head. He hasn't looked back since, willfully sentencing people to slavery, sadistic experimentation, or to be melted down into fuel for his Bio-Reactor, if he doesn't have any use for them.
In the RP I do with Lightsnake, we created a villain for our huge Naruto RP named Hiro. If its not the fact he manipulated Biri, or that he tried to off his brother Kuro, its the fact he implanted a special seal on every slavegirl in his harem in Kumogakure. So that if he dies...he can drain their life and be reborn years later.
In Clash of the Elements, Joe Dark's Vicious slaughtering of the Genesis Samurais definitely qualifies under this, more so considering how he basically spits on their honor after their deaths. This trope is even discussed afterwards by Alpha who even says straight out that "You've crossed the horizon, now we're gonna make you fall."
Cackletta Sacrificing one million lives and destroying Rogueport as the Shadow Queen just so she can prolong her own life is merely ONE of many instances where she crossed the MEH.
There is also Dimentio, who crossed this again when it is revealed that he purposely ruined Blumiere and Lady Timpani's life just to further his plans to get the Chaos Heart. What is particularly bad about this example is that this is a MEH crossed by hundreds upon hundreds of alternate universe Luigis as well.