Named for the boundary around a black hole from which there is no escape once crossed, this trope uses the black hole as a metaphor for evil; the Moral Event Horizon refers to the first evil deed to prove a particular character to be irredeemably evil.
Note the word irredeemably. It is a demonstration of permanent evil; as in, the first evil deed whose role in the story is to tell us they will always be a bad person. That moment where you know for sure that it is simply not possible for them to wash their hands to get rid of the damned spot of blood. The moment any Freudian Excuse they may have loses all meaning. And of course, many villains stay evil throughout, but we're talking "If you can find it in your soul to even consider forgiving this person, there's something freakishly wrong with you." Their existence is a blight on humanity. They. Are. Vile.
While they may not have had a term such as this to define it, many authors clearly recognized it. Robert A. Heinlein's Stranger in a Strange Land referred to it as being the result of an act that was "so bad, so black" that it was basically unforgivable. Meanwhile, multiple religions have the concept of "perdition", where those who have committed a truly unpardonable sin are irrevocably doomed to punishment in the afterlife.
Obviously, it follows from the definition that a character can't cross this boundary more than once. Crossing it implies going from redeemable to irredeemable, and that's it; the other way around contradicts the definition. Of course, that doesn't mean they'll always be getting worse. Sometimes a character who has crossed the horizon will invoke I've Come Too Far afterwards... but they have still crossed the line. Yet there are ways to stem the descent into a true monstrosity. Sometimes all that stands between man and monster is a Single Tear... or even a full-out weep. Perhaps a show of respect for the enemy. A Heel Realization that you've gone too far. Sometimes they become The Atoner. But the act has been committed, and they will never fully succeed....
And since it's subjective, some characters will think you've crossed it, while others may be still prepared to believe in your possible redemption.
Just as with a real black hole, the closer you come to a Moral Event Horizon, the harder you must try to escape.
A Complete Monster lives on the other side of the Moral Event Horizon, but crossing the Moral Event Horizon does not automatically imply that the crosser is a Complete Monster. The character can just be a bad person (and maybe even somewhat sympathetic); the Moral Event Horizon is a black mark in their history that can never be forgiven. A character who performs an act that should make them irredeemable but somehow gets away with it is a Karma Houdini. Unless they realize it, feel horrible, and work their ass off to atone for that crossing. Then maybe it'll evolve into Forgiven, but Not Forgotten; that one crossing certainly will stay as a black mark, but they're working to be a better person. Such instances, however, are rare.
Sometimes, however, there is a positive usage of a Moral Event Horizon. If, in a work, a villain seems to be too ineffectual and pathetic to be a threat, yet the show wants to insist that it's a dangerous villain, letting the villain cross the Moral Event Horizon can be a good way to establish that villain's caliber, that he's meant to be opposed and taken seriously. In other words, it can save a supposed villain from being a failure of a villain.
When a hero crosses a Moral Event Horizon and it becomes questionable whether they can still be qualified as a hero, this is Designated Hero.
- And That's Terrible: When the writers feel the need to explicitly point out when this occurs.
- Always Chaotic Evil: A race or group is treated, justifiably or not, as so innately evil that they're over the line by default.
- Bad People Abuse Animals: Characters who abuse animals are vile and soulless MEH crossers.
- Bait the Dog: A villain sets up a moment that endears him to the audience, only to commit a deed to show how utterly despicable they are.
- Bullying The Disabled: Ostracizing or mocking someone's disability is a sick act.
- Cain and Abel: Sibling Rivalry gone too far to the point that they're now at each other's necks usually ends up with one sibling killing the other.
- Complete Monster: A character that's pure evil. Not everyone who cross the Moral Event Horizon become this, but a Complete Monster has definitely crossed it.
- Cop Killer: When a murderer demonstrates his lack of regard for morality by killing a representative of law and justice.
- Crosses the Line Twice: When these actions are played for laughs.
- Defiled Forever: In a setting where Sex Is Evil and/or where Nature Adores a Virgin.
- Despair Event Horizon: Some characters who lose all hope can be more likely to cross the Moral Event Horizon.
- Dick Dastardly Stops to Cheat: Giving up an advantage just to hurt someone, while definitely stupid, petty, and pointless, can also be a Moral Event Horizon if it involves a truly heinous act due to the underlying thought process: they care less about winning and more about inflicting harm, and they're totally willing to throw away what was a guaranteed victory just to indulge in petty cruelty.
- Disproportionate Retribution: Sometimes, the retribution is disproportionate enough that it causes the person who committed it to cross the Moral Event Horizon, and often it does.
- Evil Feels Good: Being on The Dark Side after a Face–Heel Turn makes a character feel good committing evil deeds.
- Face–Heel Turn: A guy on the side of good turns bad and allies with a villain (or group of villains) and forever affiliated with same.
- The Farmer and the Viper: Rewarding selfless acts of kindness with wanton cruelty can absolutely be a Moral Event Horizon.
- Faux Affably Evil: Many Affably Evil characters can become this when they cross the Moral Event Horizon.
- Forced to Watch: Forcing someone to watch the torture or murder of those they care about is almost invariably an MEH.
- For the Evulz: Many usually Kick the Dog for no reason than Evil Feels Good and to get a sick kick at others' expense.
- Heel–Face Door-Slam: A Heel–Face Turn and/or an apology is attempted, but for whatever reason(s) is rejected; in other words, it's too little, too late.
- He Who Fights Monsters: He who seeks revenge against the villains should be wary, not to get so influenced by them that he becomes a villain himself.
- I Did What I Had to Do: When a character does something irredeemably terrible in the name of the greater good (or, less favorably, believes that it was for the greater good) while being at least nominally aware of the gravity of their actions.
- If You're So Evil, Eat This Kitten: One is made to cross the line to prove their character.
- Ignored Epiphany: A villain or morally gray character has a brief moment of realizing how bad their actions are, only to shrug it off a few moments later, knowing how far they've fallen.
- I've Come Too Far: When a character recognizes (or believes) that he/she has crossed the MEH and knows that he/she doesn't have a chance of redemption anymore.
- Jumping Off the Slippery Slope: A character skips over several progressively darker shades of gray and goes straight to the Moral Event Horizon.
- Karmic Death: What the irredeemable character will usually face due to crossing the Moral Event Horizon.
- Karmic Rape: A character may also face this if they've done something irredeemably awful, especially if that thing involved sex.
- Kick the Dog: An act which serves no purpose other than to mark the character as evil, which frequently overlaps with this.
- Knight Templar: One of the most frequent crossers of the MEH, as they're too deluded to realize that they're in the wrong or believe that the outcome will vindicate them.
- Matricide: A character killing their own mother is considered to be heinous in most cases.
- My God, What Have I Done?: The character will react with great remorse after he/she crosses the Moral Event Horizon.
- Obligatory War-Crime Scene: Soliders crossing the line to show War Is Hell.
- Offing the Offspring: Parents killing their children is considered a Moral Event Horizon.
- Omnicidal Maniac: Trying to kill EVERYTHING almost certainly puts one past the line.
- Pædo Hunt: Sexual contact with a minor is treated as a Moral Event Horizon, as it combines Would Hurt a Child and Rape Is a Special Kind of Evil.
- Patricide: A character killing their own father is considered to be heinous most of the time.
- Rape Is a Special Kind of Evil: Rape is easily considered a Moral Event Horizon.
- Revenge by Proxy: Exacting revenge on someone innocent is treated as a Moral Event Horizon.
- Redemption Rejection: A character rejecting an offer to redeem himself is proof that he isn't willing to turn back from the line he crossed.
- Reformed, but Rejected: Frequently befalls those who crossed the MEH if they try to atone. This may or may not be reasonable depending on their pre and post MEH-crossing actions.
- Rejected Apology: Sometimes, a character's actions cross the line to the point where he isn't forgiven for them no matter how apologetic he is.
- Roaring Rampage of Revenge: A character in the middle of one of these very well may do something that strikes beyond the pale.
- Sadist: A character who enjoys the pain and suffering of others and enjoys inflicting many of it as possible is a frequent line crosser.
- Shamed by a Mob: A large group of characters will show extreme disgust towards the one crossing the MEH.
- Slavery Is a Special Kind of Evil: Slavery is considered a Moral Event Horizon. Applies first and foremost for slavedrivers who start it, and not always for those raised in a slavery-based society if they aren't cruel to the slaves.
- Slowly Slipping Into Evil: One common slow approach to the Moral Event Horizon.
- The Sociopath: One of the most frequent crossers of the MEH, as a complete Lack of Empathy, overarching egotism and grandiosity, ability to feign social cues to such an extent that it appears genuine to regulars, and willingness to escalate their depravity when they've grown bored with garden-variety cruelty are a lethal combination.
- Start of Darkness: A flashback or prequel that shows where they crossed the line.
- Stupid Evil: Characters who act like this tend to cross the MEH with alarming regularity, as the behavior is inherently pointless and purposeless and they also very frequently escalate their behavior when they've grown bored with garden-variety cruelty.
- Suicide Dare: Telling someone to kill themselves is a heartless act.
- Suicide Is Shameful: Suicide is considered a Moral Event Horizon crossover by some cultures.
- Team Killer: Killing one's own teammates is a reprehensible act.
- Then Let Me Be Evil: Deeming someone as evil without having done anything evil, and after getting fed up they decide to become evil or at least act the part. If mistreated and abused for no reason their persecutors have ironically crossed the Moral Event Horizon themselves. Their first unambiguously villainous act may well be their Moral Event Horizon.
- This Is Unforgivable!: An in-universe belief that the Moral Event Horizon has been crossed. See also This Means War!.
- Tragic Mistake: The line is accidentally crossed. May or may not be irredeemable depending on how they handle it.
- Tragic Villain: A character believes they've crossed the Moral Event Horizon, and has nothing left to do but continue to be evil despite regretting it.
- Treachery Is a Special Kind of Evil: When betraying someone is considered a Moral Event Horizon.
- The Unfettered: Willing to do whatever it takes to achieve their goals, even if it means crossing the Moral Event Horizon.
- Video Game Cruelty Potential: Sometimes certain things the player can do will push their character over the Moral Event Horizon.
- Would Hit a Girl: For a man, hurting an innocent woman may be considered a Moral Event Horizon.
- Would Hurt a Child: Hurting a child in any way, shape, or form, is considered a Moral Event Horizon.
- You Monster!: Characters will get called this, indicating that they've severely crossed the line.
- Your Cheating Heart: Cheating on one's spouse or significant other is considered unforgivable in-universe.
- Adopt the Dog / Pet the Dog: An action by a morally neutral or ambiguous character that is meant to elicit audience sympathy for the character.
- All Crimes Are Equal: When all bad deeds, whether big or small, are treated as equally unforgivable, although the punisher may cross it if it is taken too far.
- Designated Villain / Unintentionally Sympathetic: The work portrays characters as having crossed the line but audiences digress.
- Easily Forgiven: Sometimes, acts that seem unforgivable are shrugged off in-universe.
- Easy Road to Hell: Similar to All Crimes Are Equal, except that all crimes automatically condemn those committing them to Hell.
- Even Evil Has Standards: A villain isn't considered purely, irredeemably evil because he is against a Moral Event Horizon even if he crossed another one.
- Everyone Has Standards: Even though they may disagree on where the line is, most people believe there are lines that shouldn't be crossed.
- Felony Misdemeanor: When something that isn't really that big a deal is treated as utterly unforgivable, although the punisher may cross it for real if he takes it too far.
- Heel–Face Turn: In which a villain turns good.
- Heroes' Frontier Step: The good deed that shows a character will become a true hero.
- Godzilla Threshold / Shoot the Dog: What would normally cross the line is portrayed as morally necessary.
- Kick the Son of a Bitch / Pay Evil unto Evil: The reprehensible act is not viewed as an MEH since the victim deserved it. It still may cross the line if it is excessive, hurts innocents in the process, or is an act of pure malice with a convenient excuse attached.
- Like a God to Me: Nearly the opposite of this trope. A character has done something so awesome that other characters fall over themselves to praise her/him.
- Mattress Tag Gag: Almost always Played for Laughs. The act of removing the tag from one's mattress or pillow is considered a deplorable and horrifying act by other characters.
- Poke the Poodle: A half-assed attempt at villainy.
- Anime and Manga
- Comic Books
- Fan Works
- Films — Animation
- Films — Live-Action
- Live-Action TV
- Myths and Religion
- Tabletop Games
- Video Games
- Visual Novels
- Web Animation
- Web Comics
- Web Original
- Western Animation
- Crankshaft. The September 2013 plot arc has Crankshaft get even surlier than usual because a new co-worker is challenging his reign of smug, curmudgeonly idiot incompetence; his response is to try to 'scare' her into letting him be the festering asshole using his hard life as an excuse to be the antisocial clod he was born to be... by trying to run her over with his school bus.
- While deaths in the ring are not unheard of, they are extremely rare. So when Ox Baker killed Alberto Torres with his heart punch, that wasn't when he crossed the horizon. It was when he bragged about the death and tried to cause more with intentional heart punches that he crossed the line. And he was hated everywhere.
- Big Boss Man crossed it by killing Al Snow's dog and feeding it to him, mocking the Big Show's father's death, and stealing his father's corpse at the funeral.
- Colt Cabana crossed the event horizon in IWA Mid-South when he tried to rape Chris Hero's student, Nadia Nyce. In fact, some of his fans declare his IWA M-S run a Dork Age solely because of this incident or just choose to ignore that he was ever there.
- Jimmy Jacobs crossed the event horizon in the Independent Wrestling Association, or at least IWA Mid-south, when he appeared in rival promotion All American Wrestling and threw the IWA M-S Heavyweight Title belt in the trash. He's since shown up in IWA M-S again, in defiance of his permanent ban but is always the bad guy. He was considered a hero in AAW though, at least initially. He also crossed it in Ring of Honor when he tried to impale his ex-girlfriend the Lovely Lacey with a railroad spike, as even stablemates Tyler Blacknote and Joey Matthews felt he was going over the line.
- Brock Lesnar either crossed it when he pushed an injured Zach Gowen down the stairs, or when he assaulted Shawn Michaels and broke his arm.
- Portia Perez crossed the event horizon when she tried to kill Allison Danger's unborn child. While she had masked babyface runs prior she was never really forgiven and retired in SHIMMER with no real friends, besides maybe Matthews.
- Seth Rollins crossed it when he took a retired Edge hostage and attempted to Neck Snap him even after John Cena gave in to his demands.
- Jonathan Coachman crossed it during his Disproportionate Retribution phase against John Cena and his father (an elderly non-wrestler) for their admittedly Necessarily Evil extremes against Randy Orton for repeatedly terrorizing them at the time.
- Randy Orton once mocked Rey Mysterio Jr. by claiming that the recently deceased Eddie Guerrero (one of Rey's best friends) was in Hell.
- Michael Cole mocking the death of Jerry Lawler's mother.
- Roman Reigns driving an ambulance with Braun Strowman inside into a production truck with a seeming attempt to murder him. And that's after he lost a match to Strowman. And he's supposed to be the face here. note
- John "Bradshaw" Layfield crossed it when he attacked Eddie Guerrero's mother at a house show leading up to Judgment Day.