Takashi Sugurono. Some consider that he was one of the sadistic teachers for the Team Summer A candidates to already push him really close to the MEH, but especially counted it when the Final Test rolled around. So when he was on-screen again during the Ryugu Shelter arc, he was already considered past the MEH so that his decision to kill part of the inhabitants, without their prior knowledge, in order to temporarily secure the continued survival for the rest of the inhabitants seemed like an unnecessary cherry on top. Other readers considered the Ryugu Shelter spoiler to be his actual MEH. Though Takashi does eventually redeem himself in some readers' eyes.
Ukai from the Team Summer A candidates crossed it when he decided that, in order to kill Ango, he would take advantage of Shigeru's naive, good-natured heart and made him lure Ango into a trap. And then decided to shoot Shigeru, because he wanted to help Ango. Fortunately, Ukai dies shortly after.
Sora Takeuchi from Air Gear proved himself an as asshole when he threatened to physically harm his ex-girlfriend Rika Noyamano —who is also pregnant with his own child.
Akame ga Kill! wakes up with one of these each week more or less, but special mention goes to Wild Hunt, particularly fat pedo-clown Champ, who murder and rape people on a bi-daily basis with the authority that comes with having the Prime Minister's son for a leader. What really sends them over the line is they do it to Bors' (a loyal empire soldier and with a decent personality) WIFE AND CHILD ON HIS GRAVE.
Tetsuo of AKIRA crossed one of these himself. Tired of having to be rescued by Kaneda and wanting independence, he felt ecstatic when he developed superpowers while in custody of the military. When Kaneda does arrive to rescue him, and military soldiers show up, he blows them away, hurting Kaneda in the process. Instead of showing any sympathy for him or sorrow for what happened to him whatsoever, he just laughed and said "Oh, that's right. I forgot you were there!" And boasted on how he won't need to be rescued anymore, even proceeding to attack Kaneda again... this time on purpose! Attacking your own friend with NO remorse!? That's just wrong. And that's even beforewhat he does to Yamagata.
A good example of a crossing of one of these that pushes you over the Despair Event Horizon is when thugs break into your house and murder your entire family, which is exactly what happens to Yurippe in a flashback in episode 2 of Angel Beats!.
Yuri herself almost crosses the line in the manga by killing all the students, starting with the first graders. The scary thing is, she would've done so then and there had Hinata not intervened before the first murder was supposed to take place.
It begins with one, as the Colossal Titan and the Armored Titan destroy Wall Maria. The massacre that follows is horrifying enough, but even a year later the situation for humanity is so grim that the government is forced to cull 150,000 refugees in a mission to reclaim lost territory to avoid mass starvation and civil war. In total, humanity lost 20% of their total population and 30% of their territory, much of it vital farm land needed to support the population. There is absolutely no doubt that the creatures responsible are pure evil. But then it is Subverted, when it is learned that Team Dad Reiner and Extreme Doormat Bertolt as well as Broken Bird Annie are the ones responsible for the entire thing and did it when they were kids. All three are fully aware of just how horrible their crimes are, absolutelycrushedby guiltand deeply traumatized over having to do it. It instead serves to make them Tragic Villains, having already committed an unforgivable act before they were old enough to fully grasp their actions and learn to care about those they were sent to slaughter.
The Beast Titan crosses it in its first appearance when it interrogates a Survey Corps Member and has him devoured by a bunch of Titans as an afterthought. Unlike the above examples The Beast Titan cracks a joke at his victim's expense. It's no wonder the Beast Titan has a 'Cruelty' ranking in the guidebook.
As a Dark Fantasy series, Berserk has many villains that perform really sickening acts. In terms of crossing the Horizon, several characters stand out.
The most notable example of this occurs during the Eclipse after Griffith crosses the Despair Event Horizon near the end of the Golden Age arc (which is about the point where the anime ends as well) and activates his Crimson Behelit. Griffith, Guts and everyone in the Band of the Hawk gets transported to the Nexus, a place that to mortal eyes looks downright Hellish, where the Godhand and every demon in the Berserk universe has gathered. Griffith, distraught over the destruction of his dream and wanting more than anything to have a second chance, does a truly malignant Face–Heel Turn, choosing to sacrifice the entire Band of the Hawk and betraying everyone he has ever led in order to become the fifth member of the Godhand, Femto. And as if all this wasn't bad enough, Griffith then goes the extra mile across the Horizon with his very first act upon being incarnated as Femto, which is to brutally rape Cascato insanityright in front of Guts, who is being pinned down mere feet away from the act after he had just made the dire decision to chisel off his left arm in a last attempt to save her. And it's made even MORE heinous in that Femto is raping her in a way that is mirroring Guts' childhood rape experience. This is the king of moral event horizons in this series, if not in all of anime. After this, Griffith acting like a hero when he is reincarnated on Earth tends to be just a little bit ironic.
Particularly because the vessel that Griffith used for his reincarnation on Earth was Guts and Casca's child, which was conceived just before the Eclipse, but which was corrupted by what Griffith did to Casca as Femto. A kid that, had all of this not happened, would possibly have been Griffith's godson. Yeah.
Gambino, Guts's gruff mentor/father figure, loses all our sympathy after he sells Guts, who was just eight years old at the time, to a paedophile soldier named Donovan for three silver coins, resulting in a traumatic rape that still affects Guts even after he is all grown up. Particularly when we learn Gambino's reason for it immediately before Guts kills him in self-defense — he called Guts "disgusting" and felt that he "can't be raised to be loyal like a dog," since Gambino blamed the kid for the death of his lover Shisu from the plague.
The King of Midland, who after ordering Griffith thrown in the dungeons and put to the torture for having sex with his daughter Princess Charlotte, tries to force himself on her out of madness. Charlotte just barely manages to fight him off, and the experience alienates her from her father, to the point where she won't even acknowledge him on his deathbed.
Inquisitor "Bloody Scripture" Mozgus is just as horrible as you'd expect a fanatical religious nutjob with too much power and not a shred of objectivity can get. While it's pretty obvious from the start that he's Bad News, when a band of starving refugees attempts to steal some of the ample foodstuffs sent to Mozgus and his retinue, he spots among them a woman with a starving infant. When she begs him to feed her child, he gently takes her along to his residence, lauding her courage and dedication. He sends away the child to be fed and cared for, then escorts her to a room while extolling the fact that while her intentions were good, she still has to expiate her sins... Then he opens the door, where we see the other refugees being horribly tortured, and the poor woman is dragged, stripped, and tied to another torture device over her increasingly frantic pleas... Then the door closes. It's as nightmarish as it sounds, if not more so.
As bad as those are the worst has to be Emperor Ganishka. If you name it he has done it: ethnic cleansing, putting POWs on the front lines, wide spread torture, attempted rape (of Charlotte, again) to more horrifying examples like turning the Midland capitol into a city of death, using black magic to conjure evil spirits to fight for him forcing pregnant women into pits of chained apostles to produce demon soldiers who tear their way out of their still living mothers and finally using an artificial means to becoming an even more powerful demon that would destroy everything. All the while bragging about making the world into hell. This guy was so evil that when Griffith faced off with him following his reincarnation, the fans rooted for Griffith.
Sosuke Aizen from Bleach has had many candidates for these. What's arguably the worst of them all? Using his powers to fool Hitsugaya into thinking he's stabbing him...when in reality, the one he's stabbing is Momo Hinamori, Aizen's own ex-lieutenant, as well as the person Hitsugaya has sworn to protect. Hitsugaya's horrified screams as the illusion breaks and sees what he has doneto Momo are EPIC.So brilliant. So cruel. Guess he'll still have his leather pants on, though. Once he becomes a deity, he decides to butcher Ichigo's home town by having his aura eradicate anyone near him, killed Gin in a brutal matter and planned to hang Tatsuki just to see Ichigo's reaction.
Many readers consider Mayuri Kurotsuchi to have crossed the horizon when he experimented/tortured Ishida's grandfather to death along with over 2000 other Quincy, and then showed Ishida a photo of his grandfather's corpse. There's also the time where he turned three subordinates of his into bombs without their knowledge.
And he's still outdone by his even moreEvil Counterpart, Szayel Aporro Ganz. Not only he takes delight on torturing Renji and the aforementioned Ishida slowly and painfully, but when Kurotsuchi and his lieutenant Nemu arrive to face him, just as Kurotsuchi is about to defeat him he grabs a captured Nemu, rapes her with tentacles to forcibly impregnate her, and kills the poor girl (one of the biggest Woobies on the whole Bleach, natch) after emerging from her body, leaving an empty husk. This is so squicky and cruel that a good part of the fandom saw the Fate Worse Than Death that Kurotsuchi dealt to himas something to cheer for.
Nnoitra does this when he hints he intends to rape Orihime at some point (he stuck his fingers deep in her mouth in the manga for crying out loud — for those who don't know, sticking one's fingers in someone's mouth during sex is a very standard kink in Japanese porn, and it had to be bowdlerized in the anime), and in the backstory, his hatred towards Nel led him to collaborate with Szayel so he could take away her powers because he was jealous of her for being stronger than him and then he kicks her out of Hueco Mundo. He immobilized Orihime in the very creepy manner described above and forced her to watch as he has his subordinate Tesla beat Ichigo to death. Thanks to a Big Damn Heroes moment by Kenpachi, however, Ichigo is safe and Orihime heals him.
We also have Big Bad Duumvirate Tsukishima and Ginjo conspiring to cross the MEH with each other's help. The first brainwashes almost everyone Ichigo cares for to turn them against him and break him in body, mind and soul, and the other sets up a Memory Gambit that involves erasing his own memories of being an enemy, to get close to Ichigo and train him up until it's the moment to break him.Horrible and brilliant at the same time.
Subverted in the case of Yukio. He gleefully claimed to have crossed this via driving his parents to suicide, but Hitsugaya quickly deduces (and shows!) that he wasn't as heartfelt and happy about it as he believed, driving Yukio to defend his parents and to a Villainous Breakdown.
The Big Bad of the final arc, Yhwach, crossed it in his backstory. In order to augment his own power, Yhwach power-drained and thus killed off the "Gemischt" Quincies (those who had partial human ancestry) and the "Echt" Quincies (full Quincy ancestry) regarded as "impure". Let us be clear on how far past the event horizon Yhwach has gone, ALL of those killed were his descendants. And that included Ichigo and Uryuu's respective mothers, and should have included Uryuu (he only survived due to a mysterious special power)...who was 8 years old at the time. So that's genocide, familicide, and attempted infanticide, all in one act. Oh, and did we mention that Uryuu's mother lingered in a coma for three months before finally succumbing?
Blood-C. Fumito Nanahara showed his true colors when he let the main cast (except Yuka) and the entire town to die in the hands of the Elder Bairns. But what really hits the marks is shooting Saya in the eye and taunting her to catch him as he leaves. And even before that, he's the one who brainwashed Saya in the first place.
Blood+: Diva crosses it when she rapes Saya's younger brother Riku so that she can bear his child, and then kills him.
Liang Qi from Canaan crosses this when she throws Hyakko through the Despair Event Horizon via tricking her into killing her savior and love Santana, and then mocking Hyakko's grief gleefully.
Hard to imagine a shojo anime like Candy Candy to be on the list but there's two in the series that are added on the list,
Eliza has several moments of dog kicking, but her worst actions are framing Candy and Terry to get Candy expelled from St. Paul School and scheming with Ruth to get Candy's reputation smeared because she took the amnesiac Albert into her household.
Code Geass is known for many morally ambigious characters (particularly with Lelouch) but there are those who stand out:
Rolo crossed it when he killed Shirley after she mentioned wanting to reunite Lelouch and Nunnally, the one thing Rolo did not want. Even now the fandom is divided whether or not his Heroic Sacrifice redeemed Rolo.
Schniezel crosses this when he leads Nina to believe that building FLEIJA would fulfill Euphemia's wishes, which directly leads to Nina's (arguable) crossing when she does build the damn thing! And before that, Nina nearly attempted to suicide-nuke the city to call out Zero forEuphemia's death. Yes, she's grieving but planning to everyone, including your classmates, with nuke has gone too far.
V.V. crossed it where he's the one who killed Marianne which sets the whole plot of Lelouch's rebellion in the first place all because of pure jealousy and even lied about it to his own brother, Charles, who later figured it out.
Light Yagami of Death Note arguably crosses the MEH early in the story when he kills the FBI agents. Afterwards, his murders just get more and more extreme until after Kiyomi Takada, his former girlfriend and now follower, kills Mello. She calls him from Mello's truck, naked, crying and terrified. He coldly tells her to continue writing as many names as she can, and hangs up. Then, as the task force is driving to find her, Light writes on the piece in his watch that the poor girl dies by burning to death, so that all the evidence is destroyed. This act of utter heartless cruelty cost Light all but the most diehard of his fanbase.
For others, it's when Light kills Naomi Misora. It's not so much that he kills her, but the way he does it (forcing her to commit suicide) and the fact that he tells her that he's Kira the moment before the Death Note takes effect, just to seethe look of horror on her face as she realizes what's about to happen. As if that wasn't enough, once the Death Note takes effect, he taunts her with offers to get her in touch with the Kira Task Force, knowing that she can't do anything about it.
Hitoshi Demegawa at first seemed to be a Slimeball TV executive cashing in on Kira. Then he leads a riot on SPK headquarters, with the intention of killing every member of the group. When Demigawa is subsequently killed by Light's subordinate Mikami, even the Kira Task Force side with Kira in doing so, unanimously agreeing that if anyone deserved to be killed by Kira, he did.
Vamdemon/Myotismon was the Digimon series's first ReallyBig Bad. If his string of Kick the Dog moments - he tortures and kills his Mooks, he captured Tailmon/Gatomon when she set out looking for her partner on her own and whipped her into a Child Soldier, he has his followers invade the entire city of Tokyo and kidnap its children to weed out the eighth Chosen One, and that's for starters - don't gradually nudge him over the Moral Event Horizon, then Wizardmon's child-scarring death - taking an attack meant to kill Gatomon and Kari, by the way - certainly does. It makes it downright beautiful when the fact that Vamdemon just made it extra personal to Tailmon and, by extension, Hikari triggers Tailmon's evolving into Angewomon and impaling him with an arrow. Even that doesn't stop him, however, and he gets even worse.
Piedmon crossed the line multiple times over. Genocide against the Digital Guardians, skewering poor Chumon, massacring Primary Village (which is filled with Digi-eggs of baby Digimon), turning the kids and their Digimon into keychains, and trying to outright murder the two youngest children by cutting the rope they were climbing are all things he did and took sadistic joy in doing.
While he wasn't as bad as Myotismon or Piedmon, Puppetmon crossed his MEH when he threatened to kill Yamato/Matt if Takeru/T.K. didn't come with him...and then tried to kill the kid with a freaking gun. And if that one's arguable, then killing one of his own underlings who was begging him to take him with him is definitely going beyond the pale.
Machinedramon's pretty vile too. His most likely MEH moment was killing the hoard of Numemon that were trying to protect Kari by blasting them all.
If you think that's bad, you haven't seen Digimon Savers. Kurata crosses the line when he specifically made a machine to destroy Digimon permanently and then used it. Upon realizing that he had made it worse, he made it even worse.
Kouki from the same series had no qualms about holding a city hostage, not caring about the thousands of lives lost. His comrade Nanami kills Digimon out of a desire to fill a lonely void in her heart, and his other comrade Ivan is doing it to get money for his family. Kouki? Does it For the Evulz.
MetalPhantomon also crossed it by using the DATS agents' worst fears and memories against them in order to suck out their life energy. Thomas got it particularly bad.
Dragon Ball: For such series known for its idealisms, it has some of the most despicable villains in anime:
Mercenary Tao crossed it for many when he killed Upa's father, Bora, for no particular reason. He then threatened Upa, saying he should be thankful that he's still alive. There is also the underline fact that he had no problem taking a contract to kill a twelve years old boy, super-strong or not.
Officer Black goes too far after he decides to destroy the entire Red Ribbon Army base just so he can kill Goku. There were still many soldiers left on the base and this comes after he gets on Red for needlessly throwing away lives.
The Crane Hermit becomes completely irredeemable after he tries to murder his own students because they refused to kill an opponent who couldn't fight back.
For some, the Pilaf gang freeing Demon King Piccolo from the rice cooker he was imprisoned in just show they can share in his conquest. Before that they had never done anything especially bad (except try to kill Goku and friends and they were laughably incompetent at doing so anyway).
King Piccolo, it's impossible to know exactly when he crossed the line. His first rampaged led to the deaths of millions of humans including Master Roshi's teacher and corrupted the Crane Hermit. As soon as he is released, he sends Tambourine to murder all the martial artists in the world so no one can used the Evil Containment Wave on him again. He directly murders Chiaotzu and destroys Shenron so the Dragon Balls can't be used against him. He then takes over the world, frees all the prisoners, and abolished all laws so he can watch humanity tears itself apart. Even with all of that, he plans to have a lottery for the thirty-six years where he will destroy one section of Earth.
King Piccolo's son, Tambourine, crosses it when he kills Krillin and brags about it to Goku. He then goes on to murder at least fifty martial artists, including a scene in the anime where he murders King Chappa and all his students in cold-blood.
Some say Vegeta crossed this when he murdered an entire Namekian village in cold-blood. Others think he crossed the line when he allowed himself to become a Majin and murdered thousands to provoke Goku to fight him.
While many of the villains do some pretty horrific things, Frieza may just top them all, between his genocide of nearly the entire Saiyan race along with his own loyal men begging to not to do it while they were there (and later, temporarily, the entire Namekian race) and his murders of Dende and Krillin, the latter of which sets off Goku's Super Saiyan transformation, leading to Frieza's undoing. The genocide of the Saiyans is especially horrible considering, while evil, they were completely loyal to him, yet Frieza killed them anyway out of paranoia that they might rebel. Which ended up being a major Self-Fulfilling Prophecy.
Frieza's Co-Dragon, Dodoria, crosses it when he kills Denda's brother, Cargo, in cold-blood. For some, he crosses it earlier when he snaps one of the Elder Namek's neck after he handed over the Dragon Ball.
Cell crosses it daily with the absorption of innocent people, slowly absorbing Android 18, destroying populated islands, has his clones beat up the Z fighters while looking cool, but he truly crosses it once he smashes Android 16's head, earning the beatdown he deserved from Gohan. To clarify, previously, any killings he had done and would do would be easily rendered mute with the Dragon Balls and all his actions were to achieve his objective or simple harmless fun. He then kills Android 16 for good, just to anger Gohan, the fact that Android 16 was never revived in a series where Death Is Cheap, only cements this. If that wasn't enough, after he murders Trunks he mocks Gohan about Goku's death, saying he died for nothing and only made him stronger. He then mockingly thanks Goku.
Cell's creator, Dr. Gero. Not only did he make a monster like Cell who drank other people to get stronger, but he kidnapped Androids 17 and 18 and forcibly made them cyborgs. He also set them up to be absorbed by Cell. And it's his petty grudge against Goku that destroyed at least two timelines, ruined Trunks' life, and nearly blew up the solar system in the main timeline.
It's hard to pinpoint where Babidi crossed the line. Is it where he brutally murders his enslaved servants who surpasses his expectations? Is when he allow Buu to eat Dubara who was nothing but loyal to him? Or maybe it's when he gleefully laughs as Buu eats entire cities. Pick one.
Super Buu going on his genocidal Human Extinction Attack in which he destroys every last human on earth, with some minor Kick the Dog moments by turning Chichi into an egg to smash it and turning the remaining cast into chocolate to devour them.
Kid Buu tops this by destroying the Earth, killing almost everyone except for Goku, Mr. Satan, Denda, and Bee. He then nearly destroy the Afterlife, that would erase everyone he already killed.
No matter which incarnations of Zamasu are there, every single one of them cross the line at some point with their "Zero Mortals" plan but each version specifically crosses the line as mentioned:
For Present Zamasu, he not only crosses it when he is about to murder his master but also when Goku tells him what his future counterpart had done, his reaction is simply joy at what his future self had done.
Future Zamasu, crosses it when he either participates in the destruction of all mortals or when he taunts Goku about how his alternate counterpart murdered his family.
But Black Goku's atrocities however eclipse both of them but the very first moment where he definitely crossed it is when upon managing to steal Goku's body, he proceeds to murder not only the now swapped Goku but also his families Goten and Chi-Chi just for the heck of it. Once Black taunts Goku for this, it made Goku go completely berserk and decides that he will never forgive both Black and Zamasu for all they had done, one of the very few times when Goku completely flipped out.
Even before she got captured by the Diclonius Research Facility, Lucy spends a good amount of time going to different peoples houses and murdering the families of those homes, just so she can have a place to eat and sleep for a couple of nights. The news even reports Lucy as a serial killer, which she was.
Although some interpretations feel that, while Kouta could never forgive Lucy, he could, and did, accept Nyuu by giving her a Mercy Kill.
Sabertooth's master Gemma, kicks out Yukino for losing a match and that's after he threw a wine glass and ordered her to strip.As if that's not enough, he then later almost erases Lecter, simply because of the fact that a cat is wearing the Sabertooth mark.
Minerva, after fighting Erza and Kagura for a while, reveals that she's taken their mutual friend Millianna hostage in order to force them to fight each other. After Kagura is essentially defeated, and prepares to surrender to Erza, Minerva stabs Kagura from behind to claim the points for herself, and reveals that she tortured Millianna while she was waiting.
Surprisingly enough, Mest Gryder, despite being one of the good guys, crosses this by implanting Brandish with Fake Memories in an effort to manipulate her into killing August, the head of the Spriggan 12 and a grandfather-like figure to her, in the middle of a diplomatic meeting to resolve the current conflict.
Yura Invel of the Spriggan 12 crosses it when he forces Gray and Juvia to fight each other to the death, hoping that Gray, by killing a comrade will awaken his inner darkness and kill E.N.D. (who happens to be Gray's friend/rival Natsu), an action that forces Juvia to perform a Heroic Suicide, since she doesn't want to kill Gray or have him live with the guilt of killing her).
Freezing has Louis L. Bridgette, the leading female's half brother. Sure, it was sick that as a kid, he had a creepy fondness for touching Satella that led her to having severe issues, and everybody in the forums were clamoring for his castration. But now? His obsession with her has exploded to the point he rapes another woman just because she looks like Satella. Seriously, WTF.
Mark Spencer, the head of the Chevalier and overseer of the E-Pandora project. He deems the E-Pandoras as something that can be easily replaced and tortures Elizabeth simply because she tried to fight against this injustice.
The Big Bad, Akito Soma, has several options, but the closest to this is how she treated the kindly nurse Kana, by blaming her for Hatori's eye injury that she caused because she was angry that Hatori wanted to marry Kana and asked for permission to do so. The Mind Rape was so bad that Kana ultimately agreed with Akito's blame of her and fell into permanent grief/madness and had to have her memory wiped by Hatori.
In Akito's and some of the fans' opinions, Shigure crossed it when he slept with Ren as a revenge for Akito sleeping with Kureno. His betrayal is somehow worse, considerin Akito's relationship to her mother (which was the indriect reason why she even cheated on Shigure in the first place.
And all those abusive parents:
Ren Sohma crossed the MEH long before the story even took place by threatening to have an abortion if she wasn't allowed to raise her baby as a male. That baby? She'd become Akito and being forcibly raised as a male would be her Start of Darkness. The worst about it? Ren's reasons: She was jealous of her unborn girl because her husband Akira was so happy about becoming a father. The ongoing rivalry (which only gets worse when Akira dies) is the main motivation behind Akito's constant puppy punting, eventually driving her to become the Manipulative Bitch that the fandom Love to Hate. Summed up, Ren's horribly selfish and cruel actions lead to almost all the crap that happens to the Somahs in whole.
Kyo's Father crossed the line when he told Kyo it was his fault his mother had commited suicide, especially because of his motivation: he felt guilty himself, for some very good reasons. His verbal abuse might have not been helpful.
Rin's parents cross it when they physically and psychologically abuse her. If this didn't do it for you, try them abandoning Rin when she collapses out of stress and is delivered to a hospital, basically telling her (a nine-year-old!) to never come back.
Yuki's mother: When she practically sold Yuki to the already unstable Akito as a "playmate", fully knowing that Akito had snapped and become abusive.
Momiji's mother crossed this when she told her son that giving birth to him was the most horrible thing that ever happened to her and that he made her sick. She even lets Hattori delete her memories of Momiji as a consequence.
It's hard to decide when Kyoko's parents did, but probably when they kicked her out when she returned home after a long stay in the hospital, due to the fact that she tried to end her own career as a gang member. It's so bad that Katsuya gives them one HELL of a "The Reason You Suck" Speech before he takes Kyoko in.
Basque Grand crosses this in the past when he coldly forces Mustang to kill Winry's parents because they were helping Ishbalans, which almost led to Mustang committing suicide over the incident.
Dante creates and maintains a military dictatorship and has her minions, the homunculi, go around stirring up conflicts and encouraging people to engage in forbidden alchemy and human sacrifice — all for one goal, to create the magic stone that will allow her to keep on living forever. There's how she kidnaps Ed's friend Rose and her baby, brainwashing the already mentally-broken Rose into an Empty Shell that will be her new host.
Pride plows right through the Moral Event Horizon at the end of the series when he remorselessly strangles his son Selim to death for bringing his remains (having no idea what they really were) to him during his battle with Roy.
We knew from his first appearance that Envy was evil and would kill you if it's amusing or he's ordered to, but one of two scenes becomes his Moral Event Horizon crossing, depending on who you are. For some, he irreparably crossed the line when he killed Hughes after turning into his wife Gracia to keep him from striking back, and triggering some of the series' biggest Tear Jerkers. And if that didn't do it for you, there's still no way you can put this guy in leather pants (at least not without people questioning your sanity) after he gleefully describes in the manga how he started the Ishbalan war by turning into a war-opposing soldier and shooting an innocent child. His absolutely cheerful description of the events that happened after he shot the child, how it ruined the life of the soldier he turned himself into, and how humans are so easy to manipulate revealed him for the monster he really is. What he said got Ed so angry that he punched Envy in the face so hard that Envy's face had to regenerate immediately afterwards.
Shou Tucker is so twisted that he turned his own five-year-old daughter Nina into a chimera by fusing her with their pet dog Alexander, creating a horrible abomination whose entire life consisted of pain that would later have to be mercy-killed by Scar. While the main reason he did this is because he was being pressured that without "interesting" results from his experiments, the government would kick him out, Tucker later admits that he "didn't have a reason", and eventually would have gotten around to doing it just to prove he could.
"The Gold-Toothed Doctor" spends most of his screentime crossing the Moral Event Horizon. He's responsible for experimenting on people to make human-based homunculi; except for the one who became Wrath/King Bradley, all of the subjects died or were rendered mindless and used as Elite Mooks, and he later slits Riza's throat in an attempt to force Roy Mustang to open the gate.
Father wiped out the nation of Xerxes just so he could absorb half the souls and gain a body.
Pride crosses this when he eats his "brother" Gluttony alive to gain his sensory abilities for a momentary advantage in a fight. Even hardened onlookers who were trying to kill Gluttony themselves at the time were shocked and disgusted.
Brigadier General Clemin declaring Mrs. Bradley expendable after she's kidnapped by Mustang.
Wrath pinning Mustang down while Pride forces him to do human transmutation. Even earlier, Wrath crosses it when he bluntly refuses Ishbalan spiritual leader Loug Lowe's offer to sacrifice his life to end the conflict in Ishbal, coldly saying that one life is only equal to one life, and that he plans on exterminating the Ishbalans anyway.
Gauron almost shooting Kaname's teacher in the face for calling him a coward.
Seina getting Kugayama to pilot the Behemoth by telling the kid, who thinks she's his sister, that she'll be nothing to her if he doesn't.
Gankutsuou: Arguably, when the Count kills Franz by mistake while trying to kill Albert in a brutal and gory way in front of his mom. The fact that Albert and Franz are barely more than children doesn't help. Interestingly, this marks a significant difference with the novel, where Mercedes not only knows about his plan to fight Albert in a duel, but begs him and eventually convinces him to spare her son
Andrea trying to rape his own half-sister Eugenie, then seducing and bedding his own mother and finally poisoning his father. And then getting away with everything like a Karma Houdini should.
And Heloïse Villefort. She seems perfectly normal in her first few appearances, with a warm motherly appearance and pleasant smile. She is the stepmother of Valentine and is the second wife of the crown prosecutor Villefort. She is the biological mother of her young son Edward, from her first marriage; however, Valentine is to inherit all of the fortune, leaving her stepmother jealous because Edward doesn’t get a single penny. So when the Count seduces her and innocently introduces her to toxicology and gives her a deadly ring which releases a deadly poison Heloïse becomes murderous and tries to poison Valentine and her husband, as well as inadvertently almost poisoning Albert and poisoning one of the servants of the Villefort household. And she does all this while maintaining her angelic motherly facade up till near the end of the series when her husband finds out what she's been doing and points out that she's nothing more than a murderous insane woman. She tries to deny his accusations before she breaks down and collapses to the ground, finally giving in to her insanity. Her husband then puts her in an insane asylum for the rest of her life. Plus the scene where she masturbated while thinking of the Count and the poison ring he gave her; in essence, she was also getting off to the thought of killing her stepdaughter and husband.
Keido may have crossed it at one of two points—either when he tried to prevent the Adam and Eve of the new world from being an incestuous couple by sacrificing hundreds of children in horrific experiments, or when he murdered his old friend Kurosu in cold blood for no other reason than seething jealousy.
Kurosu Ouma crossed it when he agreed to go ahead with the pregnancy that would kill his wife, so that his daughter could trigger the apocalypse by sexing her new brother up. About the best that can be said is that he wasn't too happy about it afterwards.
Averted by Pino in Gunslinger Girl. Right when he's about to kill Aurora for walking in on his plans one too many times, Flanca warns him to refrain from doing so. Played straight, however, by Ski Mask Guy in chapter 83 just for being the only one present besides Jose when Henrietta's bad memories of her family's death return because of him and Jose's the only one left alive present. This leads to both Henrietta and Jose dying within minutes of the resulting Freak Out, if not seconds. Dante himself is well beyond the MEH by the time he offed Enrica, her parents, and Jean's girlfriend with a single car bomb, and it's also implied due to Ski Mask Guy's presence at the Battle of Turin that Dante was also behind the murder of Henrietta's family. Oh, and as a final "fuck you" to the Agency? Only Rico shooting herown fratello could finally put an end to Dante once and for all.
Inverted by Haruhi Suzumiya in the second light novel, when she and Tsuruya spike Mikuru's drink merely for the sake of realism during the shoot for her Z-grade movie, despite all the risks it could bring to Mikuru's health. When Kyon realizes Haruhi's reason for doing it, it's a Berserk Button and nearly earns her a punch in the face... which subsequently leads to her Heel Realization when she realizes that she's doing it wrong (especially in the anime, where we have her staring away from Kyon in remorse and with tears in her eyes) and then to Character Development on her part forthebetter. As for Tsuruya herself, she subverts this with a firm grasp on the Idiot Ball, as she agreed to help Haruhi put tequila in Mikuru's drink under the assumption that Haruhi wanted to improve Mikuru's acting.
On the other hand, Fujiwara, of the Anti-SOS Brigade, definitely crosses it in the eleventh light novel with four words: "Go kill Haruhi Suzumiya!"
Hellsing: Zorin Blitz crosses it when she lays siege to the Hellsing Mansion with her forces, mind rapes Seras by delving deep into Seras' memories, forcing her to relive her painful childhood, where she had to watch her parents be murdered by gangsters and her mother's corpse being raped. While Seras is trapped in the illusion, Zorin hacks off her arm, stabs her through the back, and cuts her eyes. Bernadotte and his two remaining squad members arrive and manage to temporarily disable Zorin by shooting her multiple times and Bernadotte tries to escape with the badly wounded Seras, but Zorin kills him by stabbing him through the back with her scythe. To top it all off as Seras cradles Pip's dead body and cries for him Zorin mocks Pip calling him an "insect" and prepares to kill Seras and her fellow team members. This was enough to provoke Seras into absolutely curb stomping Zorin and her forces to death and pulling a No-Holds-Barred Beatdown on Zorin by scraping her face until it remains as nothing more than a red smear on the wall.
The burglars from Seras's past definitely crossed the MEH. Seras, who seems a cheerful and upbeat girl for most of the manga, has a rather dark and disturbing backstory, which seems to have happened when she was very young. It consists of her father being murdered by burglars, her mother hiding her in a cupboard to protect her, and her mother's subsequent murder by the same burglars. Seras tries to exact revenge by stabbing one of the guys in the eye with a fork. She gets shot for doing so and one of the perpetrators proceed to rape her mother's corpse, due to it being "still warm". In full view of Seras who lies on the ground bleeding profusely watching it happen and unable to do anything to stop it.
And Enrico Maxwell's got one too! When he started slaughtering the innocent people of London even his trusted Alexander Anderson considered him to be an absolute lunatic.
In Hi-Speed Jecy, Cross Bismarck crosses at the end of the third episode when he murders President Markham, much to the surprise of Jecy and Anita, who were trying to use the dictator as leverage against him. Jecy is quick to denounce this act of backstabbing.
Sure, that guy from the gas station did lose his mind (and explicitly admitted to being a psycho) after being forced to kill his family when they got bitten, but that doesn't excuse the fact that afterward, he tried to kill both Takashi and Rei (and rape the latter) when the two parties crossed paths. Satisfying doesn't even begin to describe how Takashi deals with the bastard.
Shido crosses it in his very first scene (if you don't count a brief non-speaking appearance in the first chapter/episode) when he kills one of his own students just before boarding the Orgybus, and he doesn't ever look back.
Shion Sonozaki torturing and murdering Satoko in the Meakashi-hen arc of Higurashi: When They Cry and forcing her sister Mion to watch. Even she admits that she's irredeemable at that point. Which is, in an odd way, why she decides to add two more corpses to the pile.
Yagi, a medium-level henchman of the final Big Bad in Holyland, crosses this by kidnapping Yuu's girlfriend Mai, drugging her, and leaving her with three of his henchmen in a situation where she could get raped easily (said drug also being an aphrodisiac) and broadcasting the whole thing live via his cellular phone to coerce Yuu into not fighting back while Yagi's thugs beat him to a pulp. Fortunately, Mai is rescued before anything serious can happen to her, and Yagi gets his due punishment from Yuu later.
His very first act was to use his shape shifting abilities to attack and nearly kill Kikyo while posing as Inuyasha, then pose as Kikyo and make Inuyasha think Kikyo was trying to kill him, all leading to Inuyasha getting pinned to a tree and Kikyo dying hating Inuyasha, which would bring massive trouble when she was revived.
When he finally steps out of his figurative and literal mask, when he reveals he revived the Demon Slayer Sango's brother, Kohaku to be his puppet and later revealed he's essentially blackmailing him into service in order to not Go Mad from the Revelation of being forced to kill his own clan and be Driven to Suicide.
Mortally wounding the revived priestess Kikyo, after becoming stronger and discarding his heart. This is the first time that he actually got his hands dirty and proving he's utterly abandoned his humanity.
Killing Kagura and forcing Kanna to commit suicide, his followers from the beginning merely out of sadism and to dispose of the heroes. Proving that not even his own followers are immune to his dog kicking.
The anime has one notable incident 15 years prior the plot, when he tries to murder a potentially powerful priestess as a child, on the small possibility she'd be a threat. This is extremely heinous even by his standards as listed above.
He ain't the only villain to have crossed it either, either. Shiori's grandfather crosses it to a horrifying degree when he killed his own son aka Shiori's dad for daring to bear a Half-Human Hybrid child and try defending humans.
Dio Brando from JoJo's Bizarre Adventure is well known for crossing the MEH over and over and over again, his very first time was at the age of 12, when he poisoned his father in a regular basis, driving him to death in a slow and painful way, as the series progresses he just gets worse and worse, even killing hundreds of people just to feed himself or for his own amusement.
Other villains from Jojo such as Kira Yoshikage (part 4) or Diavolo (part 5) are also pretty fond on crossing the line from time to time, with deeds such as killing women just to fullfil a hand fetish, or kill anybody who sees your face.
Enrico Pucci is another big example from Jojo, it didn't matter to him who he had to kill or how many lies he had to ruin to fullfil Dio's ambition of attaining heaven, the worst thing is that Pucci never saw any of his actions or decisions as morally incorrect, he truly believed he was doing the right thing all along, to quote another character from said part: "The worst kind of injustice is the evil that doesn't know it's evil"
Kaede Sakura in Kämpfer is implied in episodes 10 and 11 to have a Split Personality. Her Superpowered Evil Side had already made herself known as callous by siding with the Moderators, but when she brainwashes Mikoto and pits her against her own friends, the Purple Betrayers, in episode 13 (episode 1 of the OVA), you realize Kaede ain't coming back from the MEH.
Kenichi: The Mightiest Disciple: While none of the YAMI masters are good guys to begin with (Affably Evil, at best), resident Pencak Silat master Silcardo Jenazad is a very special case. In order to get a replacement for his slain disciple (who he'd killed himself via avalanche several arcs earlier for disrespecting their martial art), he tricks Akira Hongo, YAMI's "God Fist," into almost killing Kenichi while the other man is locked in a duel with Sakaki AND while both men are fighting even while unconscious. He pulls this off by radiating enough killing intent from behind Hongo so that the latter will instinctively lash out in self-defense and inadvertently murder Kenichi, who's right behind him at that point, in the process. And then after that happens he takes advantage of Miu's chaotic state of mind, caused by her thinking Kenichi's dead (when in fact he's survived, but just barely), kidnaps her and systematically brainwashes her into becoming a killing machine with no memories or emotion.
And just in case you don't think the above moment is despicable enough for him to have crossed the horizon, what he does in subsequent chapters should be concrete enough evidence that he's beyond any chance of forgiveness: he aggressively pits the Brainwashed and Crazy Miu against Kenichi in a fight to the death, with the expectation that Jenazad's brainwashing of Miu will be fully and finally completed when Miu kills Kenichi, or as he terms it "destroying the last piece of her heart" so that his control over her will be absolute. And for every attempt Kenichi makes to snap Miu out of the brainwashing (and any moment Miu appears to remember who Kenichi is), Jenazad urges her more and more to press on and kill the boy. It takes Kenichi taking a third option via smashing the helmet that Miu is wearing to have Jenazad's mind control of her snap..
In Kill la Kill, it's hard to say when Ragyo Kiryuin crosses this - there's her molestation of her own daughter Satsuki, the brutal beatdown she gives her in Episode 17, trying to bring about the end of the world with Life Fibers, or trying to infuse her second child with Life Fibers in-utero, and then she throws out her own daughter like garbage when the experiment seemingly fails and the baby dies. As Satsuki herself notes, she didn't even bother giving her second child a name! This was bad enough that it made both her husband and her oldest daughter turn on her and secretly plot revenge.
Deconstructed in Is This a Zombie? by Yoruno, who makes it his goal to cross the MEH enough to provoke Eu into killing him. He succeeds at the end of episode 11; before Eu subsequently kills him, he reveals that his last wish is to be reincarnated as a penguin, because he likes penguins. He himself considers the death of another of Eu's paladins at his hands to be his own personal MEH.
In the Liar Game, manga, Yokoya kills his two pet mice. Until then he could have just been someone with a traumatic childhood or someone really evil, but then he just becomes a monster.
Nanoha Takamachi always does her best to understand and help her enemies, and after StrikerS, the cast is comprised almost entirely of ex-foes. That being said, when there's a character that can't be befriended, they really can't be befriended. Precia Testarossa from Magical Girl Lyrical Nanoha crosses the line by mercilessly whipping her 9-year-old daughter Fate into a state of near-unconsciousness, and then later crushes her only reason for living.
In StrikerS, Jail Scaglietti was completely fine with anything, so long as it justified his research into cyborg bioweapons. Although he never explicitly crossed the line, his tendency to laugh maniacally while watching people suffer put him past redemption, even by Nanoha standards. His Dragon Quattro, however, did cross the line when she strapped the innocent Vivio to the throne powering the Big Bad's Cool Starship and tortured her into almost insanity to power it, and then forced her to try and kill her adoptive mother with her bare hands, while taunting them both. Quattro was punishedaccordingly.
In Force, Cypha crosses the Moral Event Horizon by admitting to killing dozens of civilians without any hint of remorse — just before almost killing fan-favorite Signum.
Grace O'Connor in Macross Frontier has a number of candidates, with conducting painful, potentially instantly fatal, and certainly eventually fatal experiments on a seven year old girl probably close to the top of the list. Made worse when they call the little girl Subject Fairy-Nine and Fridge Logic sets in with "Wait, what happened to fairies one through eight?"Even then one of the drama CDs suggests an opening for redemption with Grace when she realizes that in the ten years she spent practically raising that girl (The girl's name is spoiler:Sheryl Nome) she's developed strong feelings for her, no longer just "Fairy-Nine" but "her little fairy." She decides that the feelings will get in the way of their plan, so she deletes them with cybernetics in one of the most subtle crossings of the Moral Event Horizon ever.
In Mawaru-Penguindrum, Sanetoshi had done plenty of shit, but he seriously crossed the MEH in episode 23. He din't just do it by revealing that he was the one who "killed" Momoka via transforming her in the two Penguin Hats when she tried to stop him from destroying the world (thus causing a good part of the events of the series to take place), but by first strengthening Kanba's Face–Heel Turn via forcing him to see Masako die and then reviving her while promising to do the same to Himari if he destroys the world for him and then by setting up Ringo to be seriously injured and mocking her as she was burning to almost death, in his quest to destroy the Destiny Diary.
The Mobile Suit Gundam series has TONS of them:
In Mobile Suit Gundam, Gihren Zabi crosses it when he takes being compared to Adolf Hitler as a compliment. And then vapourises his father with a Wave Motion Gun, made worse because he also killed many soldiers on his own side and completely doomed Zeon. Some would contend that Char Aznable crossed it when he sent his boyfriend comrade Garma Zabi on a suicide mission because Garma's father killed Char's father when Garma was four.
Captain Ethan Ryer crosses it when Aina Saharin asks for a brief cease fire to get wounded soldiers out of the battlefield, he gives it... then has a GM Sniper set up and destroy the Zanzibar transport they were on.
In Mobile Suit Gundam ZZ the Dublin Colony Drop serves as an MEH for Haman Karn (who ordered it), for Mashmyre Cello (who aims the asteroid), for Rakan Dakharan (who shoots down transports and destroys bridges to make sure no one can escape) and for the EF officials (who stand by and let it happen).
Mobile Suit Gundam: Char's Counterattack, Char's Dragon, Gyunei, ends up crossing the MEH as well. At some point he defeats and captures Amuro's wingman and the local Action Girl Kayra Su, forcibly pulling her out of her cockpit and keeping her hostage in his Jagd Doga's hand. He then stages a Hostage Situation to try getting Amuro and the Nu Gundam to surrender; when the funnels of the Nu accidentally react to Amuro's anger over it, Gyunei uses the Doga's hand to crush Kayra to death, then tosses her lifeless body aside like it's nothing. Amuro, understandably, is fucking PISSED at that.
Ulube Ishikawa and Dr. Mikamura have theirs in Mobile Fighter G Gundam when they attempt to capture the Devil Gundam for their own uses, which leads to the death of Domon's mother, the imprisonment (and silencing) of Domon's father, and Kyouji becoming the Core Unit of the mutated Devil Gundam. When the end result of all of this leads to Dr. Mikamura's innocent daughter Rain becoming the Devil Gundam's core, the second in the list ends up having a Heel Realization and, having been shot by the gleeful Ulube, with his dying breath he begs Domon to save his daughter and releases Domon's father's capsule, which allows Domon's friends to rescue him.
The Frost Brothers of After War Gundam X probably reach theirs after killing off a group of people in an attempt to awaken their Newtype abilities. When they do with one person, they kill him as well out of sheer jealousy.
Mobile Suit Gundam SEED has PLENTY here. Captain Sutherland and the Earth Alliance in general by nuking their base with a lot of their allies in it, ZAFT soldiers killing POWs after said incident, Muruta Azrael after convincing the EA to fit N-Jammer Cancelers onto their nuclear missiles and promptly launching them into every hole within the meteor base Boaz... Patrick Zala's is kinda hard to pin down, though there are three good candidates - killing off the Moderates within ZAFT (including Lacus Clyne's father) because of JOSH-A's failure, shooting and having his son Athrun arrested soon after or attempting to fire GENESIS on the Earth.
Continued in Mobile Suit Gundam SEED Destiny. Lord Djibril sends the Destroy Gundam to attack European cities because they won't follow Blue Cosmos. Then comes his plan on using Requiem on PLANT colonies.
And we may have a winner here with Ali Al-Saachez from Mobile Suit Gundam 00. he starts by recruiting Kurdish kids into a jihadist army while posing as an iman. The only requirement for joining is that they murder their parents first. Then he sends several of them out into the world to do suicide bombings. And when things go sour, he just packs up and leaves them to be crushed by the enemy mobile suits. The the icing on the cake is that he is a didn't given the damn about the cause – he's an atheist who only used religion as a means to manipulate people for his own depraved goals, so to speak. And this is all before the story even started, where he gets many, many more opportunities to show how much of a dick he can be.
Not too far behind Ali we find the A-laws, who have no trouble at all with firing their orbital Wave Motion Gun at a solar elevator, to kill the rebelling forces inside, the 60.000 civilian witnesses, and the hundreds of thousands of people living below who will be crushed by the falling debris, just to prevent the populace from learning of their previous misdeeds.
Nena Trinity doesn't mess around with crossing the line as well. Out of boredom and hatred to those not playing by Trinity's rules, she vaporized a wedding in Spain (Louise Halevy was the only known survivor). With her Lack of Empathy and having zero regret over this incident, it's pretty easy to see why many fans, especially those from America, hate her so much.
Ribbons Almark himself crosses the line so many countless times. He was revealed Season 2 that he was the one who ordered the many horrific atrocities in the series from the A-laws' brutal purges to Ali's destruction of Azadistan. However, he may have crossed the line earlier when he lets Ali kill the Trinities (his bioengineered children) and kills his loyal friend Alenjandro Corner who genuinely liked Ribbons.
Gundam AGE has Desil Galette crossing the line in episode 14: for manipulating and then killing Yurin for the sake of defeating Flit, then justifying it by saying that her life didn't matter, she was nothing more than another weapon/toy for him to use, and that her involvement and death just served to make things more fun! All this from a 7-year-old child. These events lead to most of the fandom screaming for his blood.
Would you believe that AGE is the first Gundam series to have its main protagonist (Flit Asuno) cross the line? In Episode 28, when the Prime Minister was making negotiations with Vagan so they'd reach a compromise to prevent the destruction of both sides, Flit stops them. He has the Minister arrested and executed for treason and he purges anyone with any Vagan sympathies or ties in the Federation. And then he reveals his entire goal is the complete and utter genocide of Vagan! If he hasn't already jumped off the slippery slope then, he certainly has now.
The third season adds one from Zeheart Galette, ex Anti-Villain, who commands the destruction of Kio's home colony. And then there was Episode 48, where he sacrifices a large portion of the Vagan forces in an effort to destroy the Diva and Gundams. In-universe, everyone was understandably horrified.
In Muhyo and Roji, the final revelation that puts Teeki over the Moral Event Horizon is the revelation that he was responsible for Enchu's mother's death, causing his Start of Darkness and thus most of the conflict in the story. To a lesser extent, his responsibility for Rio's mother dying also applies.
Fushigi Yuugi's Nakago crosses this when he manipulates Suboshi into crossing the MEV with him by killing Tamahome's entire family, most of whom were children barely the age of ten, in what Suboshi (himself NOT a saint) believes is an act of revenge for his very much alive twin brother Amiboshi. It hurts even more when you realize that Tamahome had arrived just in time for his favorite, youngest sibling Yuiren to die in his arms, and that the second child Chuuei had tried to protect his siblings.
There's the Big Bad, Tobi aka Obito Uchiha who crossed the line in flashback during the Nine Tails' attack on the village.
Kabuto Yakushi was always a very shady figure, but had not crossed the MEH. Until he began reviving many antagonists and other powerful warriors to use them in his plans. This doesn't only include the Akatsuki members (save for Hidan), but Kages, Jinchuuriki, and at least half a dozen other extremely powerful ninja ... all of them with emotional ties to actually living characters. Like, oh, Haku, Zabuza, Shikamaru's dead sensei Asuma, and Tsunade's dead boyfriend Dan, to start...?
The Fourth Kazekage crosses the MEH with his treatment of his son, Gaara. He has a demon sealed into the boy at birth, completely isolates him from his family save for his uncle Yashamaru, who he then orders to attempt to kill Gaara and tell him that both his mother and he, Yashamaru, the only person to ever love Gaara or show him kindness, hated him to the death. Just to see what happens. Yashamaru did so against his will, and promptly killed himself to clinch it. And when the emotionally fragile, socially isolated child reacts badly to the fact that no one's ever loved him, he decides to have the kid murdered. No wonder Gaara turned out like he did, at least until Naruto intervened in his life. It's so bad that the Kazekage actually comes to admit that he was a total bastard after he's revived via Edo Tensei and Gaara dealt him a massive and magnificent call-out.
Danzo did this in the backstory, when he set up Kabuto and his mother to kill each other. Good intentions or not, that is horrifically cruel.
Madara Uchiha crossed the line when he mind-rapedKurama into being his slave, completely ignoring he was a sentient being. And the fact that it was he who orchestrated Rin's death and Obito Face–Heel Turn.
Though he was outclassed by Orochimaru in the following arc, Gato crosses the line when he brutally murdered Kaiza in front of the Land Of Waves. He also is responsible for driving said land into poverty.
Nearly everything that Orochimaru does, but the kidnapping and experimenting on multiple people, callously discarding them when they became unnessary and taking over their bodies, takes the cake.
Black Zetsu with his manipulating Indra and other Uchihas, being at least partially rsponsible for their Curse of Hatred, instigating wars and conflicts all to free Kaguya Ootsutsuki, also deserves mention.
Gendo Ikari, leader of NERV and Shinji's Jerk Ass father. Many of the things he's done through the series were questionable, but he would cross the line when, following Shinji's refusal to destroy an Angel-possessed Evangelion Unit-03, he activates the dummy-plug system to carry out its destruction, nearly killing its pilot Toji Suzuhara, Shinji's classmate and friend in the process. This is taken Up to Eleven in the manga where the pilot actually dies, and becomes all the worse still in Evangelion 2.0, where Shinji's hands are practically bolted to the controls while the dummy-plug destroys Unit-03 and almost kills its new pilot, Asuka.
To some, Ritsuko Akagi crosses it when she destroys the clones of Rei simply out of insanity and jealousy. She's deeply aware of how low she's fallen and begs Misato to kill her when she's done.
During his review of End of Evangelion, Bennett the Sage discusses this trope, positing that Shinji crosses it during the infamoushospital scene, which he finds repulsive and believes pushes Shinji out of "unlikely hero" territory and into "complete douchebag"—something Shinji even admits In-Universe.
Shinji: I'm the lowest...
In One Piece, the World Government re-enacts the Massacre of the Innocents to find the child of Gold Roger. That and the following below, will probably earn what should be the good guys in the story, a likely overthrow which is probably going to happen if the series has a happy ending.
Ohara's destruction for just having one person almost discovering the Void History.
Keeping quiet of the toxic nature of the Amber Lead in Flevance just to make more money off of the mining industry, and when a epidemic is spread they leave them to die, and slaughtering those who tried to escape. Let's face it, the higher ups have crossed the line so many times that you can write a thesis on it.
We also have the World Nobles. They're descended from the founders of the World Government, which gives them carte blanche to treat their lessers as they see fit. They are introduced to the readers when two of them are seen sneering at a slave who was wounded after trying to escape from them, kicking him and shooting him several times. Then one of them commented how his captain collection was now ruined and tells the other that she's getting a child slave, not the giant slave she wanted next.
The third one was seen riding a man and kicking him the head for not going fast enough. When a doctor and a nurse were trying to get to the hospital to bring in an injured man when they should be bowing to him, the World Noble simply kicked the injured man to the ground. When he saw that the nurse was a beauty, he promptly named her his newest wife, and when her fiancee stood up to him, he shot him and dragged the crying nurse away.
The audience (excluding the pirates) at the slave auction were just as bad. They started laughing at Hachi and saying how thankful they were that someone had shot the monster.
Vice Admiral Onigumo. He has no problems about killing his subordinate and over a thousand other marines as long as he manages to kill the pirates.
Admiral Akainu, in his debut, orders the destruction of a civilian ship, all because it might just contain an escaped archaeologist. He would then show up later to execute a lowly mook who, after he saw how outclassed he was fighting with the upper class pirates and marines at Marineford, just wanted to go home and be with his family and tried to flee the battle.
Captain Kuro spent year after year after year posing as a man named Klahadore and serving as the butler to Kaya, whose father had supposedly saved his life and let him work there. Since Kaya's father passed away, Kuro became the closest thing Kaya had to a father figure, and the two of them did many things together. Turns out, Kuro was biding his time all for the day when he could kill Kaya and claim her inheritance money, which he'd use to retire from piracy for good. The real sick part? Killing Kaya wasn't truly necessary - Kuro wanted to do it for peace of mind, because he'd secretly hated Kaya all these years, finding tending to her whims to be degrading! And he tells this all right to Kaya's face when confronted about it, leading to a big Break the Cutie moment that rightfully enrages Usopp. It's no wonder Kuro is deemed irredeemable.
Arlong after we find out that he killed Nami's adoptive mother for not having the money to pay a toll that he just imposed on the town, it looks as if he's at least willing to let Nami work to buy her town back. So he lets her join his crew and work tirelessly for 8 years to earn the 100 million berries that she needs to meet his price by going out and robbing pirates on the side. And then, when she only has one voyage's worth of money left to earn, he sends a corrupt Naval officer to steal all of the money she worked for eight years to collect. And when she confronts him about it, he has the gall to simply smile at her and ask "What promise did I break?". And then? He laughs right in her face. He later reveals that he had no intention of letting her go, and would only do so after she had drawn the map for the whole world. And considering his hatred of humans, if Luffy hadn't done anything, Nami would be let free of her association to the Arlong Pirates.
This becomes apparent with Hody Jones, a member of the Big Bad Duumvirate in the Fishman Island arc and Arlong's would-be successor. He's an even bigger racist and more merciless fisherman than Arlong ever was. He has the queen promoting peace killed and frames a human in order to cover his tracks. He planned to make a Fascist state out of his home country in order to spread these racist ideals and frequently uses his crew as meat shields (Where Arlong's love for his fellow Fishmen was his one redeeming quality). Because Hody despises humans and Fishmen who have compassion for them, after he takes over the kingdom and has King Neptune hostage, he decides to start killing all those who signed the peace petition that Queen Otohime had worked for. Judging from her success, he's willing to kill thousands. Finally, has NO reason for doing this other than blind racism.
Vander Decken IX, the other half of Fishman Island's Big Bad Duumvirate, is no better than Hody. For one thing, he willingly partnered with Hody to overthrow Fishman Island's monarchy, with the side benefit of getting Shirahoshi for himself... and when Hachi (a former ally of Arlong, who Hody held in high regard) tried to persuade them to call off their plans, what was Decken's response? He used his Devil Fruit ability to impale Hachi with arrows to the point of near-death. Oh, and his willingness to flatten Fishman Island with a big frickin' boat just because he's a Stalker with a Crush and Shirahoshi rejected him.
Following the Paramount War, a flashback shows this to be true for practically the entire city of Goa: They arrange a pirate crew to burn down the massive junkyard next to the city and everyone living in it, as the sight of refuse and poverty could offend some visiting Celestial Dragons. If this wasn't enough, they also trap the pirate crew in the firestorm outside the city walls, so they don't have to award them the noble titles they promised. They talk about all this as if it were a pleasant spring cleaning.
In the Punk Hazard arc seem to show Caesar Clown gleefully doing his latest heinous actions. That subordinate of yours that sang praises of you helping prisoners/ex-pirates like him? Kill him, he's just getting in the way. Those giant children found in your labs? Hey, turns out you've drugged them (in the form of candy) to make sure that they don't leave their island. Also, their addiction to said drug causes them to go berserk when they don't have access to it. Then there's the fact they weren't giant when he first kidnapped them. And oh golly, it turns out that if they keep on using them, they'll end up dying in 5 years and he wouldn't bat an eye.
He's also been developing horrific chemical nuclear level bombs on Punk Hazard, which he's planning on selling to the highest bidder. One of which he's going to test by deploying on Punk Hazard, murdering everyone who isn't in his HQ.
What Caesar does to Brownbeard is horrible. He admits to causing the explosion that ruined his crew, while making it seem that the latter was acting crazy and pretending to be the good guy, causing his crew to turn against him.
Once he is cornered, he uses the powers of his Gas-Gas Fruit to merge with the poisonous gas to invoke a One-Winged Angel, and demonstrates it by killing his own minions who still have faith in him.
What about Crocodile? He disgraced the reputation of both the Seven Warlords of the Sea (whom he was a part of) and, partially, the World Government (whom said Warlords worked for) by creating the Baroque Works crime organization and invading the kingdom of Arabasta, one of the few kingdoms uninvolved in the pirate/World Government conflict (formerly) for kicks, and to get his one greedy hand on a legendary superweapon, Pluton; along the way, he kills some of Arabasta's most loyal soldiers, and actually manages to defeat Luffy in combat. For this Troper, Crocodile crossed the line when, after it was revealed that he had hidden a bomb in the city that would reduce it to a crater, despite becoming its new ruler at the time, and the heroes manage to find it, it is discovered that it is a time bomb right near the end of its timer, meaning he had anticipatedthat they would find it and made sure they would still explode even if they found it. The fact that the ones who found the bomb (the princess being one of them) went down on their knees in despair and shouted to the heavens what a monster he is should say it all. Oh and he kept on sending sand storms to Toto's village, and during his first bout with Luffy, he sends a massive sand tornado to finish him because Toto was being optimistic and Crocodile felt like punishing him.
To put it in perspective, the next time Crocodile meets Luffy, in Impel Down, even though he acts "slightly" civil towards his old enemy, Cloud Cuckoo Lander Luffy just looks at him in quiet disgust and, knowing better than to trust him, outright refuses to let him out of his jail cell, only doing so when he was out of any other options (he was trying to escape with his brother Ace).
Donquixote Doflamingo always seemed to be a nasty character, but revelations during the Punk Hazard and Dressrosa arcs reveal that he does have genuine shared loyalty with his crew had redeemed him to an extent. Then we learned that during his takeover of the Riku Kingdom, he not only blackmailed King Riku into giving him the wealth of the entire nation, but it was actually a ruse to literally manipulate the King with his String-String Fruit into slaughtering his own people, intending to portraying himself as their savior. Even In-Universe, learning the full truth of this even has Nico Robin, a character whose pragmatism would normally have her advise against fighting someone like Doflamingo, declare This Is Unforgivable!.
Z was trying to have the entire second half of the Grand Line explode into a fiery wasteland, killing thousands of civilians and marines in the process. Even Fleet Admiral Akainu thought he must be stopped at all costs.
Peach Girl: Kairi has a trio of obsessive clingy jealous girls who once attacked Momo and threatened to set her on fire if she continued taking up Kairi's attention.
In Pokémon Adventures, Ghetsis was already on the line due to having raised his own son N to be brainwashed against humanity all so that he could be a puppet in his Evil Plan, but his moment officially comes after Black has defeated N. He admits to having never loved his "warped, defective freak" of a son (he has a pleasant smile on his face as he says this), then tries to have Black silenced by sicking a team of Pokemon specially trained to counter Black's team so that he's left defenseless and able to be killed on him, indicates he'll have all of Black's Pokemon killed as well, and then after he's lost, uses his associate's Pokemon to ensure that Black gets sealed inside the Light Stone while he makes his escape, mocking him as he does so! This cemented Ghetsis' status as perhaps the vilest villain of the series to date.
The point where Kodai from Pokémon: Zoroark: Master of Illusions really crosses this may vary from person to person. However, the most likely is when he freaking electrocutes a baby right in front of its mother. Unlike the other horrific things he did, this is really the first time he's shown doing something terrible that didn't have anything to do with his plan, as Zoroark stopped attacking the moment she saw he had Zorua. He even comes right out and says he enjoyed it directly after that. He probably didn't know at the time that absorbing the Time Ripple would wither the plant life of Crown City completely, but he sure doesn't give one fuck if it happens again. Furthermore, he MURDERS ZOROARK IN FRONT OF HER SON (she got better) and choked out Celebi! That said, his Humiliation Conga was extremely cathartic.
Paul from the anime itself also crossed the line. For those who don't understand, he's gone through at least three regions discarding Pokemon that fail to live up to his standards, however flimsy the excuse would be (even in comparison to how he started, which was his brother shifting gears into becoming a full-time Breeder after choking to Pyramid King Brandon). He also has little sympathy or tolerance for failure in combat, displayed in his first onscreen battle with a Starly he released after said fight anyway! It particularly crystallized during the infamous Tag Tournament, where Paul had all his other Pokemon assaultChimchar the night before the final round, had Chimchar use attacks with no regard to friendly fire during the actual battle, refused to have it healed prior to the battle, ragequit when Zangoose came out to play, and released Chimchar afterwards, who predictably ran crying to Ash. After choking to Cynthia and, again, Brandon, fans were expecting him to have some sortof breakdown after losing to Ash at the Sinnoh Conference, and took itparticularlywellwhen he didn't.
Cyrus from Diamond and Pearl arc of Pokemon anime crosses this line is when he orders Mars to blow up Iron Island, which is full of Pokemon and people living there - sporting a Slasher Smile whilst giving that order.
Pokémon Hunter J, in the "Pokémon Ranger and the Kidnapped Riolu" two-parter, really shows her cruelty when she tries to kill Ash (10-14 year old boy) several times. First she orders her Salamence to burn the surrounding forest and fire Hyper Beam directly at him, then has her Drapion squeeze him tightly, and finally ejects him from her ship at great height. She also says that she wanted to punish Ash personally and took pleasure in trying to kill him and everytime she attacks Ash she is shown with a Slasher Smile.
Damian, Ash's Charmander's former trainer crossed it when he refused to come back to Charmander (simply because he considered it to be weak) when it began to rain and Brock told him that rain would make Charmander's tail flame go out and it would die.
In a near-identical vein, Shamus (the trainer who abandoned Tepig) in Best Wishes crossed it when he revealed that he faked the affection for his Tepig and gleefully admitted to leaving it starving and tied to a pole, and that he's done this to other unwanted Pokemon too! It also qualifies as an in-universe one; as it convinces Ash that Shamus is not just a petty thug but a monster that must be defeated.
Lord Ghetsis of Team Plasma in the Episode N arc crossed the line when he ordered a mind-controlled Reshiram to incinerate everyone currently at the White Ruins...which included several members of his own team! Team Rocket even calls this out as being self-centered and deplorable.
If Lysandre's capture of the Zygarde core in the Team Flare Crisis arc doesn't do it; then he crosses it shortly thereafter when he lays waste to Lumiose City and captures Ash and his Pokémon for information on the Bond Phenomenon. Alain, who is normally The Stoic; is left horrified that he willingly helped Lysandre accomplish all this.
Xerosic crosses it when he captures Clemont in retaliation for stopping his actions at Prism Tower and tries to force him to rebuild the machinery that was destroyed by him. Clemont is able to break the mind control with a device he designed; allowing Zygarde and Ash-Greninja to stop his plans and bring him to Officer Jenny for questioning.
There are several villains in the various Pretty Cure continuities that have one:
In Futari Wa Pretty Cure Splash Star, Lord Akudaikahn freezes the Kiryuu twins in time and condemns their bodies to the bottom of the drink, and Gooyan betrays his master, revealing himself to be the main villain.
In Fresh Pretty Cure!, Cline kills Setsuna just because she ultimately saw Love as a true friend, and Northa tries brainwashing Setsuna back into Labyrinth, pressing Love's Berserk Button in the process.
In Suite Pretty Cure ♪, Mephisto subverts this when he brainwashes Siren just as she's about to see the light. Turns out, he had lost control of himself because of Noise brainwashing him, as revealed in episode 36, and Noise was the one that crossed the MEH with Mephisto's act.
In Smile Pretty Cure!, Joker is already far over the line and cements it by inflicting the Bad Ending on the Cures. And that's not going into how, in brainwashing them and Akaoni, he was behind Wolfrun's vendetta against Akane and everything she cared about, including the destruction of her charm and the subsequent attempt on Miyuki's life, and Majorina's attempted murder of two of Nao's siblings.
In Doki Doki Pretty Cure, Bel is revealed in episode 46 to have crossed the MEH when he tried to corrupt Marie-Ange, which ultimately caused her to split in two and become Madoka Aguri and Regina.
A demon lord and her minions attack a village in Problem Children Are Coming from Another World, Aren't They?: destroying things, trying to kill people, using mass hypnosis, the usual. Then Kurousagi stops the battle for supposed misconduct... which turns out to play into their Batman Gambit. The demon lord is apparently Pestilence, and she has infected most of the village with the plague, intending to delay the match for an entire month so that most of her opponents will die slowly and painfully before the battle technically begins. Just in case you were underwhelmed by the actions of the bad guys so far...
Kyubey, the Weasel Mascot from Puella Magi Madoka Magica comes close a couple of times in his Kick the Dog moments, like during the penultimate timeline in episode 7 when he manipulates Sayaka's soul gem so that she feels excruciating physical pain, just to drive home the truth about how becoming a magical girl turns you into (as Kyoko pointed out) a walking corpse by default. But he plows over the Moral Event Horizon in Rebellion; unlike in the series, in which the only way to get that vital energy was to meticulously break these girls, Kyubey had a way to fulfill his mission with the Law of Cycles. Unfortunately, the little bastard got greedy, so he decides to bring back the witch system and all the misery that comes with it, fucking over everything that Madoka did, simply because it's a bit more productive for him.
In Puella Magi Oriko MagicaKirika Kure may have crossed it when she released a Charlotte-like witch in the cast's school, which proceeded to eat Kazuko-sensei alive and slaughter many other innocent students. Oriko, a well-meaning magical girl who is herself no saint, is more of a subversion, since she uses dirty tactics only because she believes such tactics are the only way to save the world (and she truly believes that Madoka becoming a magical girl means game over for everybody). Alas, that was her own pride talking to her and blinding her to the truth—Madoka's ultimate role was not to destroy the world, but to save it. It's absolutely no wonder that in the afterlife, Oriko looks very depressed and remorseful - she likely realized how badly she fucked updue to her own pride.
Psycho-Pass has latent criminals crossing the line which makes them easy targets for lethal elimination by the bureau. But there are those who stand out:
Shogo Makishima, the Big Bad of Season 1, is known to be The Corrupter, The Sociopath and one of those responsible for Mitsuru Sasayama's death which resulted to Kougami's demotion to Enforcer. But his first onscreen MEH is killing Akane's friend in front of her after she failed to shoot him. Though that earned him some leather pants and many believed that his MEH is planning to sabotage the food supply and killing Masaoka.
Kirito Kamui, the Big Bad of Season 2, made this his Establishing Character Moment by kidnapping an Inspector and forcing her to kill her Enforcer. If that's not his MEH, then taking one of her eyes out and manipulating her to call her partner which got several hostages including the partner brutally killed just to get the Dominator is this.
Sakuya Togane already crossed the line in the past where, as a child, he killed a puppy for his mother's experiment. Then, he kills Akane's grandma with the Dominator just to darker her hue.
Kobuo Oguro may elicit some sympathy at first, as a Salaryman who gets treated like a criminal from one bad Psycho-Pass, but he loses it when he rapes his hostage.
Allegedly crossed by the high school basketball team captain by propositioning a minor, which sets the plot of Ro Kyu Bu in motion as the entire team gets punished for his alleged crime (which hasn't been proven as of the current episode), forcing the main character to coach a girls' basketball team consisting of grade-schoolers in an example of misplacedLaser-Guided Karma at work.
Hitomi Ishigami virtually blowing up the whole school with a magic mirror toward the end of the first serialization. She gets her comeuppance yet again when she's arrested for the crime. Her motive for doing this was a revenge scheme against the Newspaper Club for exposing her plot to turn the most beautiful female students into stone and use of modeling sessions as a pretense and getting her fired for it. So in other words, she wrecked Yokai Academy out of spite toward just a few of the students. It's also arguable that she may have crossed it earlier when she schemed with the Security Committee and Kuyou to murder Tsukune for the same reason.
Kuyou himself crosses it with his interpretation of the school rule "No humans allowed" as "Death to all filthy human spies!". Not only is he willing to commit murder based on his own twisted interpretation of a school rule, he's also completely willing to risk the lives of the existing members of the Security Committee by having Tsukune murdered. To make his mindset much worse, if Tsukune died and the Security Committee was eventually caught, Fairy Tail would've won.
Kanade Kamiya has a lot of potential crossing points, but it's most likely that he crossed it by killing Marin Kawamoto's husband in front of her. To make masters worse, he had no reason to kill said husband or any other humans he did at the time; he went on a mass-murdering rampage, in his own words, simply to pass the time and clear his head.
Gyokuro Shuzen is a nasty one, and her sadistic treatment of Moka just a chapter before was bad, but when she, without a single ounce of hesitation or regret, orders Kahlua to kill her own beloved little sister Kokoa? And remember that Gyokuro is Kokoa's birth mother as well, and Kahlua was so horrified by the order that she literally begged Gyokuro to change her mind.
It's possible that Hokuto and Kiria may have crossed it during the festival arc. Hokuto chains up all of the harem (with the exception of Moka who had been kidnapped) inside a barrier created by Mikogami's rosary so that they don't stop him. Tsukune tries to fight him but he keeps getting knocked down and beaten to a bloody pulp right in front of his friends. Kiria's was when he had Moka. He told her that he would let her go free if she removed her rosary by herself (back then only Tsukune could do that). To motivate her, he gave her a live camera feed on Hokuto beating up on Tsukune. Basically it boils down to, "Do the impossible bitch, or your boyfriend is gonna be paste."
Ultimately subverted with Lady Oyakata; she seems to have already crossed it by her own admission, but Ruby, her former disciple, refuses to believe it even after Oyakata critically wounds her during the boss fight and even saves Oyakata from certain death when her binding spell backfires on her. Oyakata repays the favor by using the last of her life force to sufficiently heal Ruby.
If, after all he had done and what was revealed about him, you still had a small shred of hope for Miyabi Fujisaki, that went out the window the moment he murdered Outer Moka in cold blood by destroying Moka's rosary.
Nagare Kano is already well past the MEH by the time he appears, but blackmailing a girl into bailing on the Newspaper Club for him when they're in a deadline? Putting the Newspaper Club's hard work at risk over your lust!? Calling this Kodak fiend a pervert at this point would be an insult to perverts everywhere.
In the first season, for example, Queen Beryl has many acts that could easily qualify for this trope. She caused the War that ended with many ruined lives and Endymion and Serenity's death and rebirth and years afterwards she brings Metalia into power in order to cause an Apocalypse. The way to do it is by collecting the life essence of various unfortunate civilians. She also kills Jedite and later Zoicite, showing absolutely no remorse nor shred of willingness to relent despite Kunzite's attempt to take the blame for Zoicite's loose-cannon attempt on Tuxedo Kamen's life.
Jedite crossed it earlier in the same season with his attempt on Tuxedo Kamen's life in his own last episode. Just as bad as Zoicite's aforementioned attempt, but what makes this one stand out is that he proceeds to taunt the Sailor Senshi about it. It's also an In-Universe example, as instead of doing as he had predicted, the Sailor Senshi promptly make their disgust known, both verballyandphysically. The Shitenou may not qualify however having been brainwashed by the Queen and not in real control over their actions.
In the second season, Rubeus crossed it via his extremely cruel and abusive treatment of Koan, the member from the Quirky Miniboss Squad who loves him, in which he first mocks her over losing to the Sailor Senshi, then gives her an explosive MacGuffin and orders her to practically kill herself for him. The poor girl goes into a full-blown Villainous Breakdown at that, and only Rei's compassion and efforts keep her from commiting suicide. And for worse, after Koan and Bertier have their Heel Face Turns, he goes even further than that by getting their eldest sister Petz Brainwashed and Crazy with another MacGuffin, and then leaving her and the remaining sister Calaveras to die when the MacGuffin overloads.
Played with when Esmeraude leaves Rubeus to die on his exploding ship in Episode 74. Leaving Rubeus to die when he actually seemed to trust her would be despicable... if Rubeus hadn't done the exact same thing to the Four Sisters, which makes Esmeraude's actions more an example of Kick the Son of a Bitch than anything.
In the third season, Telulu crosses it with how she deals with Mimet. To put it in perspective, Sailor Moon herself is horrified to see one of her nemeses being dealt with in such a callous fashion. Damn... when a hero is mortified by the death of a villain (who could've killed the good guys, including him/her, with a single blow, mind you) at the hands of another villain, you know the killer in question has crossed a line.
Alberich from the Asgard arc of Saint Seiya. While all the other God Warriors were following Hilda out of loyalty and love for their country, Alberich is happy to reveal he knew all along that Hilda was being controlled by Poseidon, and that he let the war go along so he'd claim all his colleagues' Saphires of Odin after they were killed, and then get Balmung's Sword and take over the world.
Dr. Katsuragi of Sakura Gari is pretty fond of doing horrible things. In the backstory he persuaded Souma (who was angry with his abusive stepmother, Sakurako) into wanting to kill her: he drugged Sakurako when she was taking a bath, took Souma there and told him to kill her as revenge for his abuse, and when Souma couldn't go through with it he grabbed Souma's hand, put the knife forcefully in it and forcibly made him cut Sakurako's wrists, effectively killing her. Then, he rapes Souma almost in the spot. When Souma confronts Katsuragi on his rape and torture of Masataka, Katsuragi denies the accusation and tries to seduce Souma. He acts uninterested and mentions Masataka's name, so Katsuragi actually grabs Souma by the throat and tries to strangle him. And then karma meets up with himSpectacularly.
Almost everyone in School Days crosses this at one point or another.
Makoto Itou: Pick your poison, but one good choice is tongue-kissing Kotonoha in front of Sekai just to hammer the point home that they're through... when she's (apparently) pregnant with his baby!
Sekai Saionji: Murdering Makoto in the bloodiest way possible, natch. It only counts because the way it's carried out it's Disproportionate Retribution for being a major asshole for much of the series. Other than that, it's more forgivable than the others' atrocities, since at least she has an excuse for it (see the spoiler for Makoto's MEH for more on that).
In the manga, killing Kotonoha and then taunting Makoto when he finds out. In that one, she has next-to-no excuse.
Kotonoha Katsura: Double murder (probably), plus the contents of her bag... Makoto's head.
Natsumi Koizumi, Kumi Mouri, and Minami Obuchi: Taking advantage of a school tradition to tape couples have sex... and then show the footage to the whole school, just for the lulz. The victims include: Sekai's friend Nanami, who gets taped and exposed by them while having sex with her sempai and boyfriend and has her reputation completely ruined among the other students., and their supposed friend Otome who is seen having sex with Makoto in that same place (which lets Sekai know that Makoto's cheating on her now), AND whose feelings they then ignore again by having sex with Makoto just for the hell of it, right after the whole festival mess.. note It should be noticed that, in the Cross Days game, it was Nanami's "schoolgirl rival" Chie Ashikaga (the sister of the main character Yuuki) who released the tape as a Take That to her. The Girl Posse's other MEH crossings remain.
In one of the visual novel routes, we also have them cornering Kotonoha, knocking down and stepping over the lunch she made for Makoto with LOTS of effort, and laughing over it. Not only that, but they even plan to treat her worse and at some point corner her to beat her up. Again, depending on your actions, Makoto can stop them, which triggers either one of Kotonoha's happiest endings (if he does) or one of Sekai's worst endings (if he doesn't).
Otome Katou: Bullying Kotonoha out of sheer jealousy and in one of the routes, manipulating Taisuke into commiting his aforementioned Moral Event Horizon via telling him that "girls like rough men in bed and they play protest-pretend while at it".
Nanami Kanroji and Hikari Kuroda: In the one of the routes, they helped Otome and her posse with their bullying of Kotonoha by spreading rumors about Kotonoha being a "pheromone girl", and also the ones who tipped the 4 of them about Kotonoha's lunchbox so the could step over it as mentioned above. And they also plotted to have the entire sports club help them make Kotonoha even more miserable. All to drive Kotonoha away from Makoto so Sekai could be with him. It backfired spetacularly on them, however, in which it ended up jeopardizing Makoto and Sekai's friendship as a whole, since Makoto's own Berserk Button is bullying and they pressed it to the core.
All of the previously mentioned MEH moments can't even compare to all of the things pulled by Tomaru Sawagoe, Makoto father: This... man is the main reason for Makoto's bad habits. His other insidious actions include impregnating his cousins, his sister, his daughters, AND his granddaughters; drugging (and almost certainly raping) his daughters so he could impregnate them when they were as young as FIVE, causing almost everyone in the School Days-verse to be related in one way or another; outright admitting to Makoto that he will steal all of his conquests for himself for shits and giggles; and simply being a horrible parent. And what's worse? Makoto, Sekai and several others are normally given swift punishment for their actions and even have some redeeming characteristics (i.e. Makoto's aforementioned hatred for bullies). Tamaru? He's a sociopathwho often get's away with what he's done! This guy is so despicable, so smug, so outright unpleasant, that even Makoto himself hates his guts. He is, without a doubt, the most loathsome character in School Days.
To a lot of people, Hatori from Sekai Ichi Hatsukoi crosses the line when he brutally raped Chiaki in the novel. The anime was nice enough to tone it down but said fans aren't going to let what happened in the novel slide and it implies it in the anime as well. Being a Karma Houdini and beating up on Yanase when the guy broke down crying after Chiaki rejected him a third time and refusing to believe that his friend has feelings for him does not help his case in trying to redeem himself.
Soul Eater: While she had committed some other terrible acts before this point, Medusa's Moral Event Horizon is generally cited as when she claimed that Crona, who she had been abusing and experimenting on for her plan to resurrect an Eldritch Abomination, is really her child; she doesn't get any better from there. Also, Giriko jumped over the line when he tried to rape Maka in the book of Eibon, when she's at her lowest point; previously, he was just another of the ineffectual minor villains.
Despite his pre-Face-Heel-Turn-reveal murder of BJ and moment of sadistic enjoyment at Kid's pain, Justin could have been seen as redeemable, especially since it was strongly hinted that Clown had something to do with his betrayal. However after he killed Tezca who deceived DWMA and put his life on line to offer a chance to return on top of friendship, it was pretty clear that Justin was not coming back to the good side. He even got a permanently burning flame on his face from the fight, in a chapter named Hellfire.
Matic in the manga version of Strider never hid the fact he was not nice, while secretly in line with the Enterprise and their Mind Control weapon. Then, he reveals to Hiryu that he set him up to kill all the figureheads of Enterprise so he'd have free way to claim their weapon, AND that he'd have the Striders' senile director killed and Hiryu and his friend Kain framed and executed for it. And even then, there's is the fact that he has known of the Zain Project for 3 years, implying that he staged the brainwashing of Hiryu's sister by Enterprise 2 years ago, which lead to her murdering 10 Strider trainees and Hiryu being forced to kill her, prompting his retirement.
Sword Art Online: Kayaba Akihiko traps 10,000 people in the titular VR MMO, making it so that death in-game will cause the death of the player in the real world as well. They remain trapped for two years, during which time thousands of people die. If that weren't enough, he inserts himself into the game in the guise of a heroic player, leading the game's biggest guild, with every intention of betraying the other players in the future and becoming the "final boss" purely to create more drama. His reason, as best anyone can figure out from what's explained, was to try and make the world of his imagination as real as possible. Kayaba is an odd case in that the source material depicts him as ultimately sympathetic, but most of the fanbase disagrees.
When the players are freed from SAO, Sugou Nobuyuki manages to trap three hundred of them in a game of his own in order to run mind control experiments on them. If that weren't bad enough, he also traps Asuna and convinces her father to let him "marry" her while she's still in a coma, gloating to Kirito that Mr. Yuuki doesn't realize that Asuna would never consent to it if she were conscious. He comes very close to raping Asuna in front of Kirito, and unlike with Kayaba it's clear that nobody is ever intended to sympathize with Sugou, who seems designed to be as repellent as possible right out of the gate.
Tokyo Babylon reveals that Seishirou is the Sakurazukamori assassin who had no qualms of killing people and is the one who put the pentacle marks on Subaru's hands in the first place. But killing Hokuto proved that he's no longer the Nice Guy that Subaru loved anymore which is so tragic.
Tokyo Ghoul gives us the following vile moments that crossed the line:
Yamori was pretty much tiptoeing the line to begin with, being a Torture Technician that straps Kaneki to a chair and begins torturing him for several days. But when he realizes that physical torture isn't breaking his latest toy fast enough, Yamori brings out his trump card. He'd captured Kaneki by promising to let his friends go, but kept the other imprisoned elsewhere. He brings up two of the prisoners, a mother and her young son, and orders Kaneki to decide which of them will die. Even Yamori's Living Emotional Crutch, Nico, protests that this is taking things too far and attempts to talk him down. This, combined with Kaneki refusing to "play along", causes Yamori to throw a temper tantrum and strangle the child to death before killing the mother.
Kureo Mado is established as a sadistic Investigator that takes pleasure in tormenting Ghouls, expressing disgust when they "imitate humans" through selfless actions or compassion. He starts things off by killing a gentle Ghoul, having made certain she realizes the weapon he's using was made from her husband's corpse. But he really crosses the line when he lures her 14-year old daughter into a trap by taking advantage of her superior sense of smell — he leaves a bag for her, which contains her mother's forearm. Once he's trapped her and her violently protective Cool Big Sis, he taunts her further by showing off his new matching set of weapons, made from the bodies of her parents. Even after all the vile things he does, Hinami still refuses to take revenge and is prepared to spare him — he takes the chance to attempt to kill her, and dies for his effort.
The Twelve Kingdoms has more than one person, specially the sen-nin or immortals, cross a few of these:
King Chuutatsu: Being cynical and upholding the rules didn't make him evil. There WAS lots of corruption in Hou, after all, and as a former government officer he knew that well. Slaughtering around 10% of the population over minor offenses, and then more after his kirin fell sick in an attempt to make things even? THAT is what made him an asshole.
Queen Kekai: Her and Chuutatsu's daughter had a nice little friend, so sweet and elegant! And wait, was she better at everything than the Princess? How dared she! So Kekai got her out of the way.... by falsely accusing her mother of accepting bribes, thus tricking her husband the King into having both mother and daughter executed. THAT would teach them!
Governor Atsuyu: He had a subordinate named Kouya, who also was the master of a certain demon. But when said kid had some people killed and eaten and was then horrified at what he had done... what did Atsuyu do? He told Kouya it's okay and had him "groomed" into a bastard who'd kill his enemies for him while he made himself look all saintly! Eeeeh...
Riyo: At some point, she took a girl from Hourai who couldn't speak the language and was completely lost... Why, Riyo immediately started humiliating Suzu by re-naming her "Mokurin" ("Fool" in the local language) and abused her psychologically and physically for 100 years straight! "Punishing her for her sins", indeed.
Shokou: He admitted he was in For the Evulz, and as such he oppressed the place he was in charge of as Governor. And not only he manipulated a kid from Hourai (who happened to be both a friend of Youko and Yuka's boyfriend) and kidnapped Old Retainer Enho (with his guards murdering his maid Rangyaku too, and the maddened Hourai kid shooting her brother Keikei almost to death), but he also intentionally ran a blinded kid named Seishuu over with his carriage, killing him, and laughed about it. What the....
The King of Kou: Okay, this one was insecure about the taika-led kingdoms like En, and then two taika girls with potential to become the Queen of Kei arrived. What did he do? He manipulated Yuka to pit her against Youko, playing on her feelings of insecurity for his benefit. And once Yuka lost against Youko, he immediately tried to kill Youko himself. No, just no.
In Umineko: When They CryRosa Ushiromiya either seriously kicks the puppy or downright crosses this when she rips Sakutaro in half. Though it seemed minor at first, you would recall that she gave Maria the stuffed lion in the first place, and that Maria considered (or made) him real, and that he was probably her only friend. She also "kills" one of the Siesta sisters, though inadvertently when she threw her on the ground. And often at times where she slaps Maria. Yes, you should show your child discipline, but when you take it too far, it is just abuse. And she often claims to have to go to work, but in reality, she goes to spend the night with one of her boyfriends — meaning she would often leave Maria home alone, often to the point where she would have to go out and get her own food. She even left one boyfriend when he told her to spend her time with her daughter. But what ultimately made her cross the horizon would have to be when she told Maria that she never loved her and wished for her to have died, so that she would have a happy life. She even tells her to go to Hell. Though, given her status as a Tragic Villain, a victim of abuse and bullying from her own siblings and her father (which Rudolf actually aknowledges), the local Chew Toy, or most likely just stressed at work, she might subvert this trope. Though, Beatrice was disgusted by her behavior.
She actually does, because Rosa is the very strange case of an abusive mother doubling as Mama Bear. In EP-2, she faces the goat servants on her own to protect herself and Maria, taking them with pens to the eyes and shotguns; in Episode 8 of the game, she shoves a rifle down Erika's throat when she mocks Maria and tells her that her mom will abandon her. Several of the episodes described here happen in different timelines (and the one described at the end happens only in EP-4), thus how abusiveor nurturing she is can seriously vary depending on which EP we're talking of.
Eva-Beatrice crosses this when she repeatedly murders both Rosa and Maria, purely for her own amusement, and in very creatively cruel ways. She really crossed it when she was supposed to murder a person once in the Twilight. She actually gets so bad that, again, Beatrice is squicked out and disagrees with her.
Bernkastel does truly horrible things, but in EP7, she shows Lion Ushiromiya the miracle of his/her existence, and then proceeds to force Lion and Ange to watch a horrible scenario in which Ange's parents are the murderers. Ange is unable to handle this, and as such she is once again torn to pieces. When Lion tries to give Bernkastel a What the Hell, Hero? speech, Bernkastel shows Lion his/her eventual fate- being massacred along with the rest of the family on October 4th and 5th, just like in all the other universes. Bernkastel laughs at Lion's fate, and she is unwilling to let Lion escape.
Kinzo also crossed it when it was revealed that Yasu, aka the real persona of Beatrice, Shannon and Kanon is actually his child fathered on his own daughter Beatrice, who he kept isolated and eventually raped because she resembled her mother, Kinzo's deceased mistress. Can you say "Just like Noah Cross"? Good.
Swordfish does this a few times in Under Grand Hotel especially him raping his lover Sen to almost death when Sen said something that annoyed him.
In Wolf Guy - Wolfen Crest Haguro Dou manipulates the younger Kuroda into going Columbine on half the student body out of vengeance against Inugami for his brother's death. Using weapons from his family gun cellar. The kicker? His brother died because HAGURO pulled his life-support.
Also, Haguro beat his subordinate Chiba within an inch of his life, biting off his tongue, and then raping him for good measure to the point that Chiba ends up dying from a combination of ruptured organs, blood loss, and broken ribs. Thankfully, Inugami gave Chiba some of his blood which ended up bringing Chiba back to life. Why did Haguro do such a thing? Because Chiba ended up encountering Inugami out on the streets and talked to him for maybe 15 minutes when Haguro wasn't around. Haguro is Yandere for Inugami's werewolf form, you see.
Wolf's Rain: Darcia crosses it when he kills Toboe. Said character is the most innocent in the story, and is still just a child at the time.
X1999. By the time Kamui decided to join the Dragons of Heaven, Fuuma joins the other side and kills his own sister, Kotori, while Kamui is Forced to Watch. The manga is more brutal that he beheads her!
Satsuki crossed it by killing Yuzuriha's dog spirit, Inuki, after the girl failed to answer one simple question.
In the movie, several Dragons of Earth crossed more lines here where Yuto and Kusanagi killing Yuzuriha and Fuuma killing his own members (including Kanoe), fatally stabbing Arashi as she watched Sorata die and pulling out the Shinken from Kotori which resulted to her death.
Yu-Gi-Oh!: Dartz from the DOMA arc reached his Moral Event Horizion when he started killing off the protagonists, but the most horrifying example and crossing of his MEH is When the identities of the Mirror soldiers are revealed to be everyone's favorite characters: Joey, Mai, Yugi, and Pegasus. From then on, he used them as frikin' HUMAN SHIELDS. This was a true Kick The Dog moment to, not only the remaining protagonists, but the entire fanbase as well. This Troper knows a few people who broke down crying because of this act.
Pegasus himself, though just as tragic as his anime counterpart, lost the sympathy of several readers of the manga when he telekinetically forced Bandit Keith to commit suicide, and would have fully crossed the line if Bandit Keith wasn't an Asshole Victim who was threatening Pegasus with a knife moments before. The anime changed this to simply having Keith dumped into the sea in a classic No-One Could Survive That, and wouldn't you know it, he does.
His treatment of the Kaiba brothers might have already put him over the line. Even Yugi determined that This Is Unforgivable!!
Another from the manga is the entire Death-T arc for Seto Kaiba. From hiring a Serial Killer who exclusively kills children to abandoning his brother, ostensibly his Morality Pet, to die, it's no wonder Yugi punished him by shattering his soul to pieces and rebuilding it from scratch; the later, Anti Heroic Kaiba who would be more familiar to those who saw the anime first is thus arguably a completely different person.
Dark Marik immediately crossed the horizon when he tried to stab his foster brother in his sleep, and then proceeded to lay down a brutal, psyche-damaging Mind Rape to Mai in their duel.
Dark Bakura crossed the line with his plan to resurrect Zorc into the mortal world in the final arc, hoping to kill everyone and cast the entire world into darkness and destruction.
Zane Truesdale (Ryou Marufuji) in the second season of Yu-Gi-Oh! GX crosses the line after donning his Dark Kaiser persona when he electrocutes and almost kills his little brother Syrus (Shou) in a duel, never giving a damn about doing that, only about winning.
Yu-Gi-Oh! 5D's: Rudger, leader of the Dark Signers, crosses it when he abandons his duel with Yusei and forces Rally, who's like an adopted little brother to Yusei, to fight in his stead. The fact that Rudger takes glee in Rally's intentional loss (which led to his death) and Yusei's anguished reaction only drives the knife further.
Yu-Gi-Oh! ARC-V: The Big Bad, Leo Akaba, is revealed to have crossed it before the series began when he ordered the Xyz genocide in order to have Ruri kidnapped in the commotion. There are several other villains who commit heinous acts but in their case... it depends on your perspective.
Quite a few YuYu Hakusho villains have fun crossing the horizon. Thankfully, all of them get what's coming to them.
Gonzo Tarukane had Yukina belittled and tortured just to produce the valuable gems her race produces. And gets killed by Younger Toguro.
Suzaku commanded all the humans under his control to hunt down and kill Keiko, then tortured Yusuke and forced him to watch Keiko and Botan get nearly killed. Yusuke nearly gives up his life energy to kill him.
Elder Toguro crosses the MEH when he brutally impaled Kuwabara over and over again, slammed him to the floor from extreme heights, then attempted to break his psyche by using his morphing powers to recreate a sickeningly twisted version of Genkai's death right in front of Kuwabara.
Mukuro's father had her fitted with cybernetics to make her a sex slave at birth, raping her since she was a child until she poured acid on herself to make herself undesirable and escaped. He also gets what he deserves, due to a combined effort between Kurama and Hiei. He is attached to a parasitic plant that will paralyze him and regenerate any wounds that he receives, and is given to Mukuro to torture whenever she wants as revenge for what he did.
Dr Ichigaki crosses the line when he poisons a famous martial artist in such a way that everyone believes he'll die of an almost untreatable illness. He gets worse when his desperate disciples ask for the cure and he promises to cure them only if they become his guinea pigs for an experiment and his team for the Tournament... but deep down he has already decided not to keep his promise. Finally, he brainwashes and transforms the disciples into heartless zombies, placing a... THING on their backs that is killing them slowly and keeping them under his thumb, and under his influence the three beat Kuwabara savagely, unable to do anything.]] Therefore, it's a HUGE relief to see Genkai managing to save the three guys, and then Ichigaki trying to pump himself with drugs to become a super fighter and getting satisfyingly curbstomped by Yusuke.
Interestingly, Yusuke himself was accused in story of that at some point. In a conversation with some good natured man-eating youkai he refers to humans as 'food'. A (relatively) normal human who was nearby said that from this moment on he has no more place in the human world and should go away. Yusuke is not evil at all, but one could argue his sense of morals was decaying by then. Thankfully, by the end, he gets better.
There's an earlier teaser in the Sensui arc. Twisted Dr. Kamiya had poisoned a hospital and continuously taunted Yusuke with the fact that due to his medical knowledge, he couldn't be knocked out: only killed. Part of the taunting was that in order to save the hospital, Yusuke would be forced to supposedly cross a MEH. Yusuke eventually does beat Kamiya lifeless, but Genkai assures him that what he did had been a grim necessity—plus she chest-compressed the just-deceased doctor back to life. Milage on Dr. Kamiya himself may vary, so his eventual turnaround may be alternately viewed as sincere or "too little too late".