Moral Event Horizon[]

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 they're 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.

NOTE: Not all antagonists who cross the Moral Event Horizon are automatically Pure Evil, as they might have some qualities that prevent them from being considered Pure Evil (e.g. The Sniper).