Once he crosses that red line, there's no turning back.
"Tarkin, if ever there was a shred of humanity in you or these twisted creatures of yours, it's dead now. You're at war with life itself. You are enemies of the universe...your Empire is doomed."
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. This is distinguished from
Kick the Dog not so much by the extreme of evil deed (though obviously said extreme of evil is relevant, as well as how unprecedented said extreme of evil is for the character) as by its demonstration of
permanent evil; as in, the first evil deed whose role in the story is to tell us they will always be as evil as they were upon crossing this - or worse. Otherwise, it was not a
Moral Event Horizon in the first place. This trope, of logical necessity, can really only be employed within the context of one story, so it's a good idea to make a big deal of it. There is certainly nothing to prevent a revision
later, if the author is so inclined.
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. Hank Rearden in
Ayn Rand's
Atlas Shrugged said that "to convict a human being of that practice was a verdict of irrevocable damnation... a
verdict of total evil" and that "he would not do so long as the possibility of a doubt remained." Meanwhile, Christian theology has the concept of "perdition", where those who have committed a truly unpardonable sin are irrevocably doomed to go to hell.
Obviously, it follows from the definition that a character CAN'T cross this more than once. Crossing it implies going from redeemable to irredeemable, and that's it; the other way around contradicts the definition. In order to cross it again they'd have to be redeemable before crossing it, which would imply that they didn't really cross it in the first place. Note that this does not mean their morality is always decreasing after crossing this; a character can cross this, then become much more horrible than they were when they crossed it, then become just a little better than they were after THAT. But they will never, ever commit an act that pegs them as redeemable.
Of course,
different people will come to different conclusions about when a character is proven irredeemable, but this is
not the same thing as crossing the
Moral Event Horizon more than once. A character can have multiple
candidates for Moral Event Horizon, but only one of them can be
the Moral Event Horizon. It's that moment where even the audience decides they can do nothing to peg them as realistically redeemable. Not even dying will do the job.
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 Realisation that you've gone too far is possible just as you're teetering on the edge of becoming a monstrosity - you've committed the act but you
regret it. But characters who regret the act that would otherwise make them cross the line must effectively become
The Atoner to 'get away' with it - just as with a real black hole, the closer you come to a
Moral Event Horizon, the more you must do to make up for it.
Can lead to a
Complete Monster, but crossing the
Moral Event Horizon does
not automatically imply a
Complete Monster. Can also turn a sympathetic character into a
Woobie, Destroyer of Worlds
Compare
Jumping Off the Slippery Slope,
This Is Unforgivable.
Contrast
What Do You Mean It's Not Heinous?,
Adopt the Dog,
Poke the Poodle,
Crowning Moment of Heartwarming,
Pet the Dog,
Like a God to Me.
Compare/contrast
Easily Forgiven (acts that sometimes would qualify for this trope are forgiven by characters anyway).
No real life examples nor any real life example section, PLEASE.
Fictional examples:
