In-universe, A Brother's Price has killing a man. Men are amazingly rare in that world, kept secluded and protected, so finding one dead is shocking.
Alan Blunt of Alex Rider. Some of his actions are questionable to begin with, but in the ninth book, Scorpia Rising, he springs head-first over the line by arranging a school shooting in order to coerce Alex into taking his next mission. Said shooting hospitalizes Alex's best friend and Secret Keeper Tom. It doesn't help that what he does leads Alex into a trap set by Zeljan Kurst.
Yu planning to have Alex painfully tortured by forcing him to donate his organs to black market clients.
Razim crosses the line when he kills Jack Starbright and forces Alex to watch. This might perhaps be the moment where Julius crosses the line too, as he's the one pulling the trigger on Razim's orders and gleefully rubbing it in.
Napoleon of Animal Farm: his definitive crossing of the Moral Event Horizon, the moment when you know he has become no better than Farmer Jones, the animals' original oppressor, is when he sells Boxer, the most hardworking and loyal of all the animals on the farm, to the knacker because he is injured and no longer able to work in a cruel and heartless You Have Outlived Your Usefulness moment.
Alongside being killed and failing to save their planet, crossing this is one of the Animorphs greatest fears.
Jake: "He's right. We have to win."
Rachel: (Narrating) "I know how the others think of me. I know that I sometimes... get too involved in the killing. But even I know that the words 'we have to win' are the first four steps down the road to hell."
A specific example would be Sixth Ranger Traitor David's murder of Jake and Rachel's cousin. Up to that point, David has made multiple attempts on the lives of the kids, but has still been portrayed in a relatively sympathetic light due to what he had gone through. But when he unplugs a sick teenager just so he can steal his identity, he finally crosses the line, and both the narrative and the kids stop portraying him as anything other then a villain.
More examples: Alloran releasing the quantum virus on the Hork-Bajir world. He's disgraced and condemned by everybody from that point forward. His brother Arbat follows suit later on, though his plan is ultimately thwarted.
Jake finally crosses the line by sacrificing his cousin to kill his brother, mass-murdering thousands of helpless Yeerks via spacing and manipulating a pacifist android to make it all work
Ax threatening to nuke the kids' hometown and the Yeerk pool with it in order to make Visser Two back down. Though the gambit works it's implied he would've actually done it. Nevertheless his relationship with the other Animorphs is permanently damaged.
Chapman selling out humanity to the Yeerks in the Andalite Chronicles. In the main series he's more sympathetic, being a voluntary Controller to protect his daughter but the deed remains, even if no one remembers it.
In general both Yeerks and Andalites have unforgivable offenses that would qualify an individual for a MEH (by their standards). For the former it's sympathizing with a host race and for the latter it's breaking Seerow's Kindness, the law that prohibits giving other species Andalite technology. Even Elfangor is not above this with most Andalite officers preferring to place the blame on Ax in order to maintain Elfangor's reputation.
Count Olaf of A Series of Unfortunate Events crosses the line with his habit of abusing children (both mentally and physically), blackmailing, murders and attempted murders of numerous people (if we count in those who he burned to death), multiple hoaxes and kidnapping of at least three people, while one of them being about 2-years old toddler. And who knows what else he got up to before the books.
In-Universe, the narrator implies that Count Olaf crossed the MEH when he slapped Klaus in the first book.
Tywin Lannister crosses this before we even properly meet him. We hear in the backstory how he order the sacking of King's Landing which killed hundreds, but it gets worse when Tyrion tells the story of his first wife. He secretly married whom he thought was a crofter's daughter whom he and his brother Jaime had rescued from being raped. When Tywin found out he told his son that she was a whore hired by Jaime to give Tyrion his first lay. To prove it, he had her gang-raped by a barracks full of Lannister guardsmen, even paying her a silver piece for each one, and then forced Tyrion to go last. It's made even worse when we later find out it was a lie. She really was a crofter's daughter who fell in love with Tyrion. When he learned this, Tyrion flew into a murderous rage and killed his father.
Joffrey Baratheon-Lannister, who decided to execute Ned Stark, the Decoy Protagonist of Game of Thrones and viewpoint character, despite promising his girlfriend and Ned's daughter, Sansa, that he would be merciful and allow Ned to join the Night's Watch. He then proceeded to become The Caligula at age 13. It could also be argued that Joffrey's MEH came much earlier, when his lie caused the deaths of Sansa's dire wolf Lady and Arya's friend Mycah.
Gregor Clegane's MEH came before the start of the series, when he murdered a baby by dashing its head on a wall, then raped the mother with the boy's brains still on his hands. And this is even putting aside the fact that when he was a child, he found his little brother playing with one of his toys... so he held his face in a burning brazier until half the flesh melted off.
One of the most chilling MEH's of the novels comes when we find out what happened to Ramsay Bolton's wife: He locked her in a tower with nothing to eat. They found her with no fingers and blood around her mouth.
Once upon a time, Walder Frey and his brood were obnoxious hillbillies that the rest of Westeros had to tolerate because they held a major crossing. Then they decided to massacre Robb and Catelyn Stark and the Northern army at the Red Wedding. Now readers cheer when random Frey children and grandchildren end up killed and used as the filling for delicious pies. This doubles as an in-universe example - to the people of Westeros, the Freys were justified in betraying the Starks and in killing them as the Starks had broken a treaty with them and dishonoured their house. It was the method - betraying Sacred Hospitality - by which the Freys did it that attracted such hatred.
In-universe, the two crimes that the people of Westeros see as crossing the moral event horizon are kinslaying and breaking guest-right.
Also played with early in Game of Thrones when Jaime Lannister pushes the young Brandon Stark out of a window for witnessing him and his twin sister, the queen, having sex. This seems at the time to be a definite moral event horizon, but Jaime later becomes a gradually more sympathetic character after he undergoes extreme suffering over the course of the story, which prompts definite change in his moral character for the better.
For many characters in-universe, Jaime crossed this line before the series even started, when he killed the very king he had sworn to protect. Jaime, however, considers it completely justified, since said king was mentally unstable and Jaime killed him to stop him from slaughtering a city of innocents (or as innocent as anyone gets in this series, anyway).
Theon Greyjoy proves just how powerful a MEH can be. He allows Reek to kill two Small Folk children and their mother along with a handful of other people to cover up the escape of Bran and Rickon, and spends the rest of the series paying the horrific consequences.
Jim Taggart, Orren Boyle, Dr. Ferris, Wesley Mouch, and Mr. Thompson from Atlas Shrugged were just really annoying Obstructive Bureaucrats to start with but they cross the line with the Orwellian Directive 10289, a bill they pass preventing all originality, innovation and creativity, essentially destroying the common man's chances for success. After that they just keep going with Project X and torturing John Galt. Jim Taggart breaks down during the torture scene however so Even Evil Has Standards. Similarly, one of the Directive's drafters, Mr. Thompson, tried to oppose any plan to kill or torture John Galt.
Jim Taggart starts off as a Woobie, Destroyer of Worlds but takes a flying leap over the Moral Event Horizon when he cheats on his adoring wife, Cheryl with the vile Lillian Rearden because he's jealous of Cheryl's moral and intellectual superiority and when she discovers them, lays a savage Reason You Suck Speech on her that surpasses Dorian's own to Sybil Vane. This results in Cheryl having a complete nervous breakdown while fleeing through the city in an absolutely gothic sequence that culminates in her comitting suicide. At this point, most readers are thinking "Die Jim, die."
Ivo Corbière from the Brother Cadfael novel Saint Peter's Fair has already stooped to murder to get his hands on a list of Empress Maude's partisans to give to King Stephen, so that he, Corbière, can win an earldom at least. Emma has it, and Corbière crosses the Horizon when he threatens to rape her to get it—and enjoy doing so. The readers cheer when Emma, in resisting him, knocks the brazier over and he burns to death.
This is how Beringar regards Renaud Borchier, alias Cuthred's betrayal of his liege in her darkest hour in The Hermit of Eyton Forest
In Catch-22, the character Aarfy is portrayed as a bumbling fool, more of a constant nuisance to the protagonist Yossarian than anything else. Throughout the book the reader is given very small glimpses and hints that he may be more than a little odd in the head. This finally culminates in Aarfy raping and murdering a woman, and getting off unpunished.
One could also point to Milo Minderbinder. He's a sleazeball war profiteer from the beginning, but he really crosses the horizon when he arranged for his own base to be strafed. He is rather smug and amused by the incident, too.
The strafing thing started after he finished blowing up the base, which would allow him to once again sell his overstock cotton at a profit. He also gets the dead man in Yossarian's tent killed and tries to get rid of all that cotton by making the other officers eat it, while fully aware that you can't eat cotton. And he revealed this plan to Yossarian during a funeral. Needless to say, he eventually get the cotton sold to Germans - to the enemy. On the condition that they strafe their own base.
There's also the time towards the end of the novel, when Yossarian confides in him that Nately has finally won the heart of the girl he loves but is distraught because he's finished his mission quota and might get shipped home without her. Milo goes to their commander and arranges for himself to start getting assigned missions (and thus a chance to earn some shiny medals). But since he's far too valuable to actually have his life risked, other officers will have to do them for him, and wouldn't you know it, Milo just happens to know that a certain officer named Nately is looking for more missions to fly. It's while flying one of those missions in Milo's place that Nately is shot down and killed.
The Christopher Pike teen horror novel Chain Letter 2 is all about invoking this trope. Each of the protagonists is given a task to complete which will push them over the horizon. If the task is not completed, the character in question will be killed, effectively giving each of them the choice between death and damnation. The tasks given ranged from the truly horrific ( Kip's was to set his younger sister on fire and burn her right arm off) to the Felony Misdemeanor ( Brenda cutting off her own finger and delivering it to one of the other characters was definitely a moment of Squick, but it's hard to see it as something worthy of eternal damnation).
Whether or not you think Thomas Covenant crosses this in the first book of the Chronicles when he rapes Lena is down to personal opinion. If you do see it as the MEH, you'll probably stop reading there.
In Chung Kuo, rebel leader deVore crosses the moral event horizon in an infamous scene and never looks back
High Lord Kalarus of Codex Alera charges straight across this and never looks back. He spends most of his first appearance finding inventive ways to be a sexist pig and belittle slaves, and his second involves attempting to kill a couple of 17-year-olds because his Smug Snake son tried to kill them and failed, and he doesn't want to look bad because of it. But we only really get an idea of how disgusting the man is in the book after that, when we learn what he did and is doing to Rook. If the fact that he was holding her 5-year-old daughter hostage isn't enough to make you hate him, the fact that he later keeps Lady Placida under control by setting gargoyles to kill the aforementioned 5-year-old if she tries anything should definitely do it. He also had his Legions target orphanages when he attacked another city, just to draw the defenders out. That 5 year old girl? His illegitimate granddaughter.
Invoked in-universe in the Coldfire Trilogy. In his backstory, Villain ProtagonistGerald Tarrant wanted to make a Deal with the Devil for immortality, but the entity he was bargaining with demanded he commit the worst act he could imagine in order to "sacrifice his humanity"- which he did by cold-bloodedly murdering his wife and children whom he genuinely loved. In the end, he drags himself back across by sacrificing his previous identity both physically and spiritually, effectively turning himself into a new person.
"Comrade Death": Hector Sarek starts as merely an unscrupulous businessman in an immoral industry. Until he Murders the Hypotenuse and lies about his role to the widow. He then attempts to convince her to marry him, if not for love then for his money. After she rejects him, Sarek tells her the truth of her husband’s death and fully embraces the concept of "death merchant".
Several characters in the Destroyermen walk the border and a number goes past it.
Don Herman crosses this when he kills his servant, who is a 14 year old nude female slave, in cold blood. Some people may consider when he traps Fred Reynolds and Kari, mutilating both of them to do his bidding is the defining point.
Kurokawa reaches this point after allowing his own men to go to the butchers for himself to live.
Nicodemus Archleone comes off as Affably Evil and portrays himself a Noble Demon... :except he gleefully tortured Shiro to death for the sole purpose of gaining more power. At this point, while he's still very affable and polite and erudite, there's no doubt that he is not sympathetic at all. A later book in the series reveals that he actually crossed the MEH centuries ago. By unleashing something terrible on the world. Even if you haven't read the books, you've probably heard of it. It's called the Black Plague.
Also in the Dresden Files, the Wardens believe that any breaking of the Laws of Magic constitutes crossing the MEH, as the Warlock will time and time again fall back to their law-breaking ways; even the character Harry was dating implied that she and her fellow Wardens commiserated the day he was spared, since all Warlocks are destined to be repeat offenders. From what we've seen with Molly and Harry they might not be wrong about this.
The villains of Changes, Arianna Ortega and the Red King, cross the line soon after they are introduced. Arianna kidnaps Harry's daughter Maggie (massacring her foster family in the process) to use her in a sacrificial ritual. The reason she wants to do this? Harry's grandfather killed her asshole of a husband, who she hated. She just felt that Harry and Ebeneezer had insulted her. She finally crosses it for good when she not only announces that she plans to go through with murdering a child in a few minutes, but that it's essentially "just business."Her daddy the Red King manages to be even more repulsive; at first he appears to just be a Caligula-style junkie, even helping Harry out by ensuring that he can duel Arianna in a situation where he has a fighting chance. Then, he reveals that it's essentially a massive act; he can actually speak perfect English which means that his reactions to Harry's insults were all staged. He then tries to sacrifice Maggie himself just to gain the prestige Arianna would have gained. And he's the one who orchestrated the Red Court system, meaning that all of their atrocities (which include centuries, maybe even millennia) of slavery, murder, and torture of the people of South and Central America are his doing.
Achilles from the Ender's Shadow series kills out of the most psychopathic need to prove his own superiority to his victims. He enters the Moral Event Horizon as soon as he kills Poke. But before he kills Poke, and to any character who doesn't know about his killings, he seems normal enough that the people worried about him killing someone appear to be the paranoid ones.
In the Eternal Champion novella, the human military commander played near it when he killed the Eldren commander while under a truce. But, the main character Ekrose crossed this firmly when he killed the human race to protect the Eldren.
In Featherless Buzzards, the abusive grandfather feeds their grandsons' cute little dog to the pig he has been feeding in the whole book. At least he gets his comeuppance and is killed by said grandsons by pushing him into the pig pen, when he is immediately Eaten Alive by the pig. Death by Irony, indeed.
Starting from Dark Moon in The Firebringer Trilogy, the once honourable and noble unicorn king Korr starts getting...a little crazy. At one point, he charges two innocent mares, with the clear intent to kill at least one of them. But he truly crosses the line when his own daughter steps in front of him...and he doesn't so much as falter.
The God of Small Things: Baby Kochamma manipulating the twins into lying to the police to completely vindicate her from the blame of Velutha's death.
Drake from Gone probably crossed it offscreen before we saw him, but when he happily goes off to kill an autistic four-year-old, and we get into his head and see how delighted he is with the prospect, there is no going back. Fortunately, he is unsuccessful.
Caine probably crossed it when he was too apathetic to stop a bunch of coyotes from feeding on young children when all it would've taken to stop them was asking them nicely. An alternate one would be his treatment of Diana in Plague, which, although not anywhere near as bad as what he'd done before, was destroying the one thing that kept him human.
Diana herself sees cannibalizing Panda as her own MEH, but, seeing as it partially prompted her Heel-Face Turn, possibly not.
Percy Wetmore in The Green Mile. Being an obnoxious prick who hides behind his connections in a Depression-era Georgia prison? There were probably a few of those types back then. Killing a prisoner's pet mouse on the eve of their execution? Harsh, but luckily, it got better. Making it so said prisoner would be roasted alive in the electric chair as payback for laughing at him? There we go. Good enough for not only the guards to put him in a straitjacket and lock him in a storage room, but for John Coffey to risk his life using his healing power to punish him. And the anvil that hits him immediately afterward was a nice touch.
In the Harry Turtledove novel The Guns of the South, the AWB crosses this when they gun down Robert E. Lee's family in the middle of a crowd. The fact that they would be so single-mindedly callous turns the Confederacy against them.
Dumbledore invokes this trope himself in relation to Voldemort, describing the creation of a Horcrux as "moving beyond the realm of what we might describe as 'usual evil.'" Considering the implications of tearing one's soul apart, this is probably justified. The fact that it takes a human sacrifice to make the process work also plays a large part.
Also crossed when Bellatrix, Barty Jr., and two other Death Eaters subject Alice and Frank Longbottom (Neville's parents) to a Fate Worse than Death - prolonged torture by Cruciatus curse until they became insane, unrecognizable vegetables. It's even worse that they did it after Voldemort was gone.
Wormtail is an instructive example of where true event horizons lie. While betraying Harry's parents to Voldemort, supposedly his best friends, causing an explosion that killed a dozen innocent people and pinning his mass murder on Sirius, ensuring Sirius' twelve-year psychological torture in prison was certainly evil, he could have still been redeemed. But after he found Voldemort in Albania and helped restore him to full health, and killed Cedric, he had crossed the line permanently. After the initial betrayal he at least had a meager chance to start the life anew, probably in another country. When he resurrected Voldemort, he truly cemented his place in darkness and ensured that the casualties would be much worse than those he caused himself.
Vincent Crabbe, along with Goyle and Malfoy were mostly ineffectual rivals to our trio, but towards the end Crabbe actually almost kills the three, even when Draco is dead set against it.
In the first book, the killing/drinking the blood of a unicorn is considered to be this, in universe.
Use of the Avada Kedavra, Cruciatus and Imperius curses is considered this in universe, since they are known as unforgivable curses and punished with a life sentence in Azkaban. In Deathly Hallows, though, use of at least the Cruciatus and Imperius curses by the protagonists during Voldemort's regime is more a sign of necessary pragmatismon their parts than anything (and in the case of Harry doing a Cruciatus on Bellatrix, it's more or less a case of Laser-Guided Karma, as she had earlier tried unsuccessfully to provoke him into doing a Crucio on her and taunted him when he failed to get the desired result).
Good luck guessing when Uncle Vernon crosses the MEH, though it was probably in Chamber of Secrets when, after Dobby's meddling causes Harry to get a formal warning from the Ministry of Magic by snail mail for violating a rule against underage wizards using magic outside of Hogwarts, Vernon takes advantage of it by locking him in his room with full intent to keep him from returning to Hogwarts whether or not he broke the same rule again.
If that's not sufficient, then it's Vernon's attempt to kick Harry out on the street the instant he learns that the enemies who've been on the hunt for Harry might actually (gasp!) pose a danger to anyone else in his household even though he knows his nephew will most likely be murdered in cold blood if he's left without a safe place to stay. Only a Howler from Dumbledore convinced him to back down.
The Masadan commander of Blackbird base takes a flying leap over the horizon, in The Honor of the Queen, when he orders the rape and torture of all his female Manticoran POWs. Honor has to be restrained from killing him in cold blood, although all this does is save him for a Grayson noose.
Steadholder Burdette and his allies cross it when they sabotage the construction of a habitat dome, killing dozens of innocent schoolchildren, in Flag In Exile.
In-universe, merely working for Manpower Incorporated is viewed as one by most Torchers, Manticorans, or Havenites.
Cordelia Ransom crosses this in In Enemy Hands after figuring out a way to Loophole Abuse interstellar law to let her have Honor executed instead of treated fairly as a prisoner of war. This act sparks no fewer than fourHeel Face Turns among the Havenite cast, who wind up either actively participating in Honor's escape, covering up the fact she survived the attempt, or going on to completely topple the Committee for Public Safety.
"Operation Raging Justice", an attack on the Manticore home system by the Solarian League following the extremely damaging Yawata Strike, becomes this for characters in-story. It marks the point where the Manticorans stop playing around and bending over backwards to try and convince the League they're in the wrong. They still make a serious effort to avoid serious bloodshed, but this time they do so by trying to force the Solarian fleet to surrender rather than simply let them withdraw. It's also the final proof for Beowulf that the Solarian League has no concern about its own Constitutional law, as a 2nd prong of the attack is used to maneuver them into a bad political position.
For Esteban Garcia in The House of the Spirits it probably happens when he molests, and likely rapes, Alba when she is still a little girl. But if that isn't enough he helps to organize The Terror. During that which he find her again. This time we know for certain he that he rapes and tortures her and at least threatens to let his men rape her as well.
In Death: Rapists will automatically be considered to have crossed this. Murderers (unless they are in the group of Sympathetic Murderer) will be considered to have crossed this as well.
In Legend, Commander Jamerson was already portrayed as someone suspicious, but it's only near the end of the first half of the story is where her true colors are revealed. Case in point, She orders her men to murder Day's mother. While Day is watching.AFTER saying that no innocent people would be killed. Is it any wonder that June decides to save Day after this?
The famous Swedish Millennium trilogy has many line crossings both gruesome and realistic:
Niles Bjurman from The Girl with the Dragon Tatoo - who is Lisbeth's legal guardian and caretaker - crosses this line either when he forces her to perform oral sex in exchange for the money she needs to replace her computer, or when he violently sodomizes, rapes and tortures her.
Zalachenko from The Girl Who Played with Fire crossed it when he beat Lisbeth's mother so badly that she suffered a crippling cerebral hemmorhage.
Even though he's trying to screw up Lisbeth's life, Fredrik Clinton from The Girl Who Kicked the Hornet's Nest crosses this when he arranges for Mikael to be murdered and framed for dealing drugs in an effort to destroy his credibility. Wadensjöö even calls him on it, saying that Clinton will end up destroying The Section because of his actions.
In Modern Tales of Faerie there is an interesting example: Roiben, a noble knight of the seelie court, is mystically compelled to obey the commands of the sadistic unseelie queen. Her idea of a good time is forcing him to do what would normally cross the Moral Event Horizon or make a Sadistic Choice.
Invoked at several points in The Monk, but when Ambrosio makes his Deal with the Devil, it's obvious that according to the rules of the story, he's gone too far.
In Richard Wright's Native Son, Villain Protagonist Bigger Thomas is from the beginning kind of a sleazeball, what with committing indecent exposure and feeling up an unconscious girland accidentally smothering that girl to death while trying to keep her from waking up and crying out, but he truly vaults over the line when he rapes and murders his Only Sane Woman girlfriend once she becomes a liability. Wright's point is that the true monster here is the corrupt system that allows people faced with crippling poverty to become this bad.
In Michael Crichton's second-to-last novel Pirate Latitudes, the Governor's new secretary, Robert Hacklett, first takes over the island and throws Captain Hunter in prison after his return, but crosses the Horizon when he allows his wife to be raped, right in front of him. At least he gets his due when said wife shoots him in the groin with a flintlock pistol.
In Rainbow Six, one member of a group of Basque separatists seeking to spring prisoners from jail coldly murders a Littlest Cancer Patient on live TV. No one really objects, though Ding does give a perfunctory dressing-down, when one of the team's snipers puts a round through the killer's liver so that he bleeds painfully to death rather than taking a Instant DeathBoom, Headshot.
The main villains also definitely count for trying to kill almost everyone on the planet by means of a modified Ebola virus in order to preserve the environment. Makes it eminently satisfying when Clark organizes a Karmic Death for them.
In Remote Man, if you don't think Frank Laana has crossed the line with his wildlife smuggling operations, you will when he beats the crap out of the teenage protagonist in a parking lot. While Ned has been investigating his activities for some time, the most Laana has to go on is that he looks like some kid he talked to for two minutes in an art gallery in the Northern Territory, and that for some reason he was sitting in the Concord Prison reception area. Aside from that, we are told of a particularly brutal smuggling job in which he had drugged a large number of birds to transport in a small suitcase. The drugs wore off too early, and the results were not pretty. It's this story that ultimately keeps Ned from giving up his plan.
In Robots and Empire, Kelden Amadiro and Levular Mandamus are already skirting pretty close when they plot the acceleration of the radioactive decay of uranium and thorium in the Earth's crust over a 150-year period, but then, at the moment when the plot is ready to be executed, Amadiro insists on turning the dial to 12, which would kill hundreds of millions, if not billions, within 20 years. Mandamus is suitably horrified by Amadiro's attempt to fulfill his quest for revenge by trying to commit genocide.
Vicar Zhaspahr Clyntahn is the Grand Inquisitor of a Corrupt Church in the Safehold series. That alone should tell you all you need to know. He and his Inquisition find new ways to cross the Moral Event Horizon every book. The standout, which is a Horizon In-Universe as well as out, is in the fourth book. He arrests and tortures to death several vicars whose only crime was being part of a Reformist circle who wanted to rein in the Church's corruption. He then goes after those vicars' families, those vicars' assistants, and their families. This includes children as young as twelve. The children he doesn't kill are shipped off to very strict, orthodox monasteries. This act is so monstrous that it turns the threat of Holy War, which had loomed over the story since the second book, into an afterthought.
The School For Good And Evil Sophie first crosses this when she murders The Beast for cutting her hair. She does it again when she breaks her promise to Agatha to arrange to kiss Tedros, in favor of wanting to stay and try to become a Princess. She crosses it a final time when she arranges for Tedros to find Agatha, whom he loves, in her arms at the Evil Ball. Of course, how much of this was of her own volition is questionable as it's implied that some, if not all of her Evil behavior may be due to the Head Master's control of The Storian.
Sisterhood Series by Fern Michaels: A number of villainous characters are considered to have crossed this by the Vigilantes. Senator Webster from the book Payback is an interesting example. Maybe you don't consider his actions of unknowingly giving his wife Julia Webster AIDS to be crossing this. Maybe you don't consider his actions of cheating on her with multiple women to be crossing this. However, the minute he, in a drunken rage over the fact that his affairs are being broadcast live, goes wife-beater on Julia is the minute you know he has finally and truly crossed this!
A number of readers are convinced that the Vigilantes themselves crossed this in Vendetta. John Chai, son of the Chinese ambassador to the USA, had drunkenly hit-and-run Barbara Rutledge and her unborn child, killing them both. He then pulled a Karma Houdini with Diplomatic Impunity. The Vigilantes decide that the best punishment for the guy is to skin him alive! Certainly, he was a creep, and was being used by the author to personify Yellow Peril, but his deeds simply did not warrant that level of Disproportionate Retribution! Not only that, but the Vigilantes just shrug off what they've done afterwards.
The attempt of the necromancers to kill over 3 billion people to get immortality,
In backstory, Mevolent killing Skulduggery's family in cold blood and torturing skulduggery to death,
And at the end of Kingdom of the Wicked, The Reflection!Stephanie killing Carol, just to test how her weapon works
In Star Trek: A Time to Kill, Prime Minister Kinchawn crosses it rather early, after he uses his illegally-acquired weapons to shoot down 10 Klingon ships in orbit of Tezwa, killing 6,000 warriors. If this didn't represent his crossing the line, his casual willingness to see millions of Tezwans killed in a Klingon counterstrike, including his own family, certainly does. What makes it worse is his apparent self-image as a Well-Intentioned Extremist, when he's really Drunk with Power. He sees his own children's death as merely a means to acquire more sympathy and thus more support and power, and seems to truly believe this is somehow reasonable.
Darth Caedus, the villainousJacen Solo, was apparently intended, to be morally grey at first, sliding down into worse and worse acts of Necessary Evil until the Evil overwhelmed the Necessary. It didn't really turn out like that, considering what he did, including fridging his own aunt, bombarding throwaway planet Fondor after they had already surrendered, and lighting decidedly NON-throwaway planet Kashyyyk on fire from orbit. Fans lost all sympathy for him long before this was intended to happen.
Interestingly, what his family considered to be his Moral Event Horizon was comparatively minor, using a Nightsister Blood Trail to track Jaina to the Jedi's secret base.
In-universe by Sith standards, his MEH would be killing Mara Jade. It cements his commitment to the dark side via sacrificing something he loves and only after does he take his Sith name. Also it's a safe bet that any fans left by that point would surely desert.
Also from the Star Wars Expanded Universe, we have an in-universe example with Kyp Durron, a young Jedi who in the Jedi Academy Trilogy gets influenced by an ancient Sith spirit to steal a superweapon out of the heart of the local gas giant and go on a spree with it, causing supernovas which kill the populations of various planets. He then flies to a training camp planet supporting about twenty-five million people where his brother had gone to train, was told by an Obstructive Bureaucrat that his brother had been killed during this training, and fired a nova-causing missile at the sun. Then it turned out that the bureaucrat had simply lied, and the brother was flown over to try and stop him, but it was too late; the only survivor in the system was Kyp, safe in his superweapon. Later the main characters found him and convinced him to stick the superweapon into a black hole, which almost resulted in his death; instead he lived, recovered, and went back to training at the Jedi Academy. Because the worlds he'd killed had been Imperial worlds, and he felt bad about killing his brother, and he'd supposedly been possessed by a 4,000 year old Dark Lord who made him do it, all was forgiven. Later books called him on it and called it hard. He'd been influenced, not possessed, or he would have actually killed Luke Skywalker instead of knocking him out. These had still been people who, as the Fix Fic type novel I, Jedi says, had had nothing in any reality to do with him. It became something he could never live down, sometimes making him The Atoner, sometimes making him tired of being reminded of something he did as a teenager when he was in his forties, trying to be a respectable member of the Jedi Council. Some characters are never able to forgive him.
Thrackan Sal-Solo crossed the horizon in the eyes of the peoples of the Corellian system, especially the Selonians, by holding his first cousins once removed hostage (as leverage on their mother, the Supreme Chancellor) and then trying to vape them. note The peoples of the Corellian system hold the Mama Bear and Papa Wolf in very high honor.
The destruction of Alderaan is an in-universe Moral Event Horizon for a number of characters. It caused a lot of Imperials to defect to the Rebellion, which even before then was largely composed of people who had been Imperial citizens or soldiers at some point. They accepted this new influx, even knowing that some of these ex-Imperials had fought against and killed them. After that, though, ex-Imperial recruits were regarded with more suspicion, many Rebels wondering why they hadn't left the Empire earlier, like right after the news about Alderaan got out. Staying in the Empire's service became a subjective Moral Event Horizon; the longer someone had been with the Empire after Alderaan, the less moral they were seen to be.
This is a plot point for how other characters treat Baron Soontir Fel in the X-Wing Series, who left almost a year after the Emperor died, and who had been the Empire's most dangerous pilot in that year. Wedge Antilles trusted him instantly, and a pilot who had survived being shot down by him similarly welcomed him, but almost everyone else either was slow to warm up to him or outright refused to trust him. He killed too many Rebel pilots and didn't see what kind of monster he served until far too late.
In Allegiance, we see that while the viewpoint stormtroopers were just as shocked by the reports as anyone else, official Imperial policies were confused, some saying that the Death Star had been hijacked by Rebels, some saying that the planet had been populated by Imperial sympathizers, some saying that Tarkin had gone power-mad. Sure, the Rebellion had its own claim, but the Rebellion was a terrorist organization, and while they were starting to think that the Empire had some deep flaw, they didn't see any better alternative. Until their unit was sent to slaughter a village, and later one of them was threatened by an officer because he aimed to miss unarmed civilians.
In Death Star, we have a personal example in Tenn Graneet, head gunner on the titular superweapon, who for most of the novel has his character built up. He always thought the Death Star would never really be used on a living planet, just on really big ships and bases and the like. When it comes to it, he follows orders. He realizes that as word gets around, even people serving with him on the Death Star treat him strangely, and knows that someday everyone will know, and everyone will loathe him as both the biggest mass murderer of his or possibly any time, and as someone who always, always followed orders. Unusually, and unlike Tarkin, who gave the order, he sees his action as a Moral Event Horizon, thinking that they would be right to hate him and one day kill him. The guilt doesn't let him sleep, and he knows he will be commanded to do worse — if he doesn't he'll just be killed for disobedience and they will get another gunner and he will do it — and, when they are in range of Yavin and his hand is at the final button, he desperately stalls while telling everyone to "Stand By," hoping that something would happen to stop him. And it did. Poor bastard. If he ever had a chance at redeeming himself, this would be it—his successful attempt to stall the destruction of Yavin long enough to allow Luke Skywalker to blow up the Death Star would be a spectacular example of Redemption Equals Death.
In the current series, Fate of the Jedi, Daala initially seems fairly reasonable -wrong, but reasonable- about the Jedi and their role in the Galactic Alliance, especially considering the actions of Jacen Solo. She even shuts down her "Jedi Court" when the parents of one of the Jedi that went berserk revealed that it's head judge was using the imprisoned jedi as wall art. Then, in Allies, she attempts to force the Jedi to bend to her will and surrender all Jedi that have snapped (despite the Jedi Temple being far better equipped to hold a mad force user.) by laying siege to the Temple with a Mandalorian Battle Fleet, with orders to "do what is necessary." The Jedi respond by sending out the Grand Master's personal assistant, a young apprentice (on the grounds that nobody could possibly misconstrue it as an attack, but she has the standing in the order to show good faith), wearing no armor, carrying no weapon, intending only to negotiate. The Mandalorian Commander, after ascertaining that she is neither of the Jedi he was sent to "arrest", calmly informs her that "My orders make no provision for negotiation." and pulls out his sidearm and shoots her down without warning. He then proceeds to announce that if the mad Jedi are not turned over promptly, he will order his fleet to vaporize the temple, and that anyone who tries to leave will be slaughtered without warning. Daala's response, on seeing the LIVE BROADCAST TO THE GALAXY, in which troops operating under her direct orders shot a teenaged girl down in cold blood and then threatened to massacre thousands of people? "Good. Now they should take me seriously." These words make her administration look like a terrorist organization.
The Yuuzhan Vong race in the New Jedi Order series goes about crossing the MEH wantonly. Aside from the killing off of many major characters, some of their things involve going against their word and destroying a planet's ecosystem despite losing a contest for its fate, intentionally attacking/destroying civilian targets in order to burden the New Republic with billions of displaced refugees, spreading a lethal disease among civilians, breeding a toxic animal specially designed to butcher Jedi, and butcher hundreds of Jedi, many young adults and teens, sacrifice millions to their Gods, as well as horribly mutilating and exploiting anyone who joins up with them.
Joruus C'baoth cements his status as an Ax-Crazy monster when he Mind Rapes General Covell into a mindless extension of his own will, and reveals his plan to do the same to the rest of the Empire.
The original C'baoth has a more mundane one- Force choking Thrawn after the latter disables his ship. Though things had been getting increasingly worse that moment is described as his true fall to the dark side.
In Star Wars: Revan, it's quickly established that the Sith Emperor was a really bad guy and he does a lot of horrible things. But the sole defining moment? He kills T3-M4 by vaporizing him. He does this right in front of Revan, making Revan watch as he murders one of his closest friends out of sheer spite.
Path of Destruction: A couple to choose from, depending on your point of view. Bane realizing he killed his father via the dark side strips him of the Force for a time. Killing rival apprentice Sirak is probably the most mundane example though it is presented as the moment Bane fully embraces the dark side. Killing an entire family including children and leaving the father for last just so he can feed on his suffering is probably the top contender. Though the thought bomb could also count- betraying his allies and dooming the souls of all the Sith and a hundred Jedi to thousands of years of unspeakable, unending agony.
Rule of Two: As Bane's apprentice Zannah also gets her pick: using Sith sorcery to drive a woman completely insane so that she tears her own eyes out until her consciousness is completely shredded aside from a small part that lives on in a corner of her mind to be tortured by subconscious fears for the rest of her life. And all that for simply having romantic relations with a past lover (whom Zannah was merely using anyway). Or more likely, brutally slaughtering Caleb after the latter healed her master and using the aforementioned spell on her own cousin so he would take the rap with the Jedi, thereby concealing the existence of the Sith.
Dynasty of Evil: Serra, Caleb's daughter haunted by memories of Bane and the grief of being recently widowed spirals down into darkness culminating in capturing Bane and using her father's knowledge to chemically imprison and torture him. Only realizing what she's done after her best friend is murdered she accepts her fate with her father's stoicism and is killed by the Huntress.
Darth Malgus despite his obvious commitment to the dark side, conflict and destruction crosses the Moral Event Horizon when he kills his lover Eleena for being his weakness. Ironically it's a Jedi avoiding that same pitfall that causes him to do this. Aryn Leneer, seeking revenge for the death of her Master plans to kill Eleena to hurt Malgus, but pulls back at the last second realizing what it means. This forces Malgus to acknowledge that his love can be used against him and in true Sith fashion kills the last good part of him.
Jefferson Pinkard remains a sympathetic character for amazingly long in Harry Turtledove's Timeline-191 series, despite being a member of the Nazi-equivalent Freedom Party, as we've known him since long before he joined and understand exactly why he's bitter enough to do it. At most, the reader is probably hoping for a while after he joins that he'll realize the path he's on before it's too late. However, when he comes up with a way to mass murder black people using truck fumes, the line is finally crossed definitively.
The "population reduction" is a Moral Event Horizon, not only for Jeff, but for everybody else involved, from Jake Featherston (another formerly sympathetic character) on down.
Melkor's destruction of the Two Trees, murder of Finwë and theft of the Silmarils. After this, he can never again take a form that looks anything other than completely evil, and is named as Morgoth, the Dark Enemy of the World.
Fëanor, after seizing the ships of Teleri and causing the Kinslaying of Alqualondë.
In the Akallabêth, after the Silmarillion but before the events of the Lord of the Rings, Sauron crosses it when he engineers the destruction of Númenor by corrupting its king; like Morgoth, after this action he can never again take an appearance that is not evil.
Saruman in The Lord of the Rings crosses the Moral Event Horizon at the very end, when he does everything possible to destroy the Shire out of pure spite. Up until that point, he'd done plenty of awful things, but had continually been offered (and refused) opportunities for redemption. The destruction of the Shire shows that he's irrevocably fallen from a wizard who was once great and wise to a bitter man with nothing left but hatred and the desire to harm others as much as possible.
While never a morally upstanding guy, Túrin is one of the few sympathetic characters in Middle-Earth to pass this, at the climax of The Children of Húrin when he murders a lame man in a fir of rage, leaving even himself so disgusted that he commits suicide.
In The Tomorrow Series , Major Harvey looks like nothing but a Jerk Ass at first...but rapidly speeds past the Moral Event Horizon when it's revealed that so far from being a member of La Résistance, he's an enemy collaborator, before advancing to full Quislinghood. He takes the lead in interrogating Ellie and her friends, doesn't protest at all when they're condemned to death, and it's no fault of his that they escaped. Nobody who reads the books feels sorry for his eventual fate.
In Warrior Cats, Scourge claims that his was killing a cat for the first time. He says that when he did it, he got a cold feeling in his belly, and it just got colder and colder and never warmed up again... and he welcomed it, as it made it easier for him to earn respect as a fighter.
Brokenstar pretty much danced across the line when he began kidnapping kits and turning them into Child Soldiers, forcing them to fight enemy warriors easily three times their size. The ones who survived the "fights" usually died of their wounds.
Tigerstar's MEH is pretty hard to find, but the biggest contender would probably be how he murdered the sweet-hearted Brindleface, fed her body to his dogs so they could get a "taste" of cat blood, and then set them on Thunder Clan. Where, by the way, his own mate and children were living.
Breezepelt crossed it when he tried to murder Poppyfrost and Jayfeather, the former of which was pregnant and the latter a blind medicine cat.
Darkstripe feeding his own half-sister—who was just a kit at the time, mind you—deathberries to keep her quiet about his upcoming betrayal.
Ashfur trying to murder Squirrelflight's father and adopted children to get back at her, over being dumped.
The Libertines in 120 Days of Sodom are perhaps the most sadistic characters in classical literature. They kidnap several people, including their own daughters, and subject them to 120 days of violent, nightmarish psychological, physical, and sexual torture just For the Evulz. They go as far as to disembowel pregnant women and maim their own daughters violently, and the author treats the characters as heroes with minor quirks!
The author in question is none other than the Marquis de Sade, whose very name gave us the very word "sadism." And there's quite a bit more where that came from — in the Sade novel Philosophy in the Bedroom, Eugenie crosses the Horizon with the horrors that she, Dolmance and the other libertines visit upon her own mother, Madame de Mistival, who came to try to rescue her from her corruption, up to and including having her raped by a man with syphillis and then sewing her genitals shut so that the polluted seed will be kept inside, which will most likely lead to her death.