Heroic Suicide

Suicide is generally considered a bad thing. Even when the person committing it feels he has no other choice, it's still far from heroic. In contrast, Heroic Sacrifices are almost always regarded as noble, and those who perform them are held in high regard. Despite the similarities, these two things rarely overlap.

But sometimes they do.

This trope is about situations where suicide becomes heroic. Say a character discovers that he is a Person of Mass Destruction, a crucial part of an Evil Plan, or the Big Bad's Soul Jar. In such situations killing oneself may be the only way to save the day.

This is a specific sub-trope of Heroic Sacrifice. The difference is that in most Heroic Sacrifices dying is merely an unfortunate by-product of a heroic act. In a Heroic Suicide, dying is the goal. Or, to put it another way, a Heroic Sacrifice engages in lethal activity even though it will get him killed. A Heroic Suicide engages in lethal activity BECAUSE it will get him killed.

The clearest examples are when a character personally will become the threat that endangers others, and they have to kill themselves to prevent it. Any such situation definitely counts as a Heroic Suicide. Otherwise, it might count, but be careful.

A good rule of thumb is that in most Heroic Suicides, the character's death is both necessary and sufficient to acomplising their immediate goal. That is to say, they can't accomplish their goal without dying, but how they die doesn't matter much. To elaborate:
 * Necessity: Ask yourself if there's even a theoretical possibility that the character could accomplish their goal without dying. If there is, it's probably a regular Heroic Sacrifice. For example, say you drink poison intended for someone else, trade places with a man on death row, or turn your spaceship into a guided missile. None of those qualify, because in each case your death is only a side-effect of what you are doing. The point is to prevent the other person from drinking the poison, buy time for the man to escape, or disable the enemy ship. You would still accomplish these goals even if you discovered you were miraculously immune to poison, received a last-minute pardon, or were beamed off your ship at the moment of impact. This trope only comes into play if the character has to die to accomplish their goal. Though that isn't to say their death will always be permanent.
 * Sufficiency: Ask yourself how important the circumstances are under which the person dies. Do they accomplish their goal just by dying, or does it matter what they were doing when they died? For example, say you get yourself killed diffusing a bomb. In that situation, it's stopping the bomb that makes your actions heroic, not the dying. You couldn't achieve the same goal just by, say, shooting yourself in the head. On the other hand, say you are the bomb. In that case, it doesn't matter how you die, as long as you do it quickly. Shooting yourself in the head is fine. So is getting poisoned, stabbed, or decapitated.

Compare Thanatos Gambit, My Death Is Just the Beginning, Seppuku, and Better to Die Than Be Killed. Often overlaps with Cyanide Pill which could be considered a sub-trope (examples go there, not here).

Warning: This is a Death Trope. Expect lots of unmarked spoilers.

Comic Books

 * Hartigan from Sin City kills himself so that no one will hurt Nancy to get at him.
 * At the end of Watchmen, Rorschach tells Dr. Manhattan to kill him so that he won't reveal that Ozymandias was behind the destruction of New York.

Film

 * In Terminator 2: Judgment Day, the T-800 ally must kill himself at the end to destroy the last possible source of Terminator technology that could let people reverse-engineer Terminators and Skynet.
 * Constantine. Angela Dodson's identical twin sister Isobel commits suicide when she realizes that the Big Bad plans to use her as part of a ritual to release Hell on Earth. She sends a psychic message to Angela from beyond the grave to tell her to find Constantine, and leaves a message for them so that they can stop the Big Bad from performing the ritual on Angela.
 * Ripley falls backward into a smelter to stop the Alien gestating in her from birthing and starting the whole ordeal all over again in Alien 3.
 * In Gran Torino, after his previous attempts to stop a gang from harrassing his neighbours failed, Walt Kowalski taunts them so that they will kill him. Thinking he was grabbing a weapon, they gun down an unarmed old man in front of an entire block worth of witnesses and are sent to prison.

Literature

 * In Harry Potter and The Deathly Hallows, lets Voldemort kill him after discovering that, and that Voldemort cannot be killed as long as he survives.

Live Action TV

 * Dawn tries to pull one of these in the fifth season finale of Buffy the Vampire Slayer, when she realises that a gateway to hell can only be closed if her blood stops flowing. She is saved when Buffy realises she can achieve the same effect by killing herself in Dawn's place. Buffy's sacrifice could thus also be thought of as an example.
 * Heroes
 * Like Dawn, Peter Petrelli considers killing himself to keep from blowing up New York. Also like Dawn, he is saved by the last-minute intervention of his elder sibling, though that's not an example of this trope.
 * Eden also performs one when she blows her brains out to prevent Sylar from getting at them.
 * On Angel Darla (a vampire) stakes herself so that her and Angel's 100% human son can be born.
 * In the Doctor Who episode "The Waters of Mars", the Doctor changes history to rescue someone doomed to die, which causes him to go totally A God Am I with megalomania. His rescuee kills herself in order to stop him, after which he snaps out of it.
 * on FlashForward learns that in the future an innocent woman will die due to his actions, so he jumps off a building so as to Screw Destiny so it can never happen.
 * In Sherlock Moriarty invokes this  Also an inversion in that