Marge Simpson: That poor man, I hope someone does kill him.
Something horrible has happened or is happening to Alice. Her pain is unbearable. Even her loved ones look away and cover their ears, trying to block out the sights and sounds of her agony. There is nothing they can do to ease her pain or save her. But they can offer one last mercy to their friend: they can end it all for her now, before it gets any worse.
Whatever is afflicting Alice is usually in the category of a Fate Worse than Death (thus making the death a kindness). There may be a Body Horror that nobody can figure out how to destroy. Or Cold-Blooded Torture has left her horribly maimed and screaming incoherently. Maybe she's being Eaten Alive. Or perhaps she's about to succumb to The Virus or The Corruption, and she wants to Die As Herself. It may be that she could survive, but they have no access to medical care; or that Alice, if she lived, would be reduced to a shell of her former self. Whatever the reason, this trope implies that death is the merciful option, even if Alice is begging for her life (or is so overcome that she cannot speak at all).
Instant Death Bullet is likely, and justified in this situation: the killer has no difficulty getting to a position and attacking in a manner that would cause quick death. There is usually a moment when a glimmer of self-awareness allows Alice to show her gratitude through a tearful smile or the like. This is usually easier when the shooter is Cradling Their Kill.
Contrast Cruel Mercy, which aims to do the exact opposite. Compare the combat-ending Coup de Grâce, the villainous Thwarted Coup de Grâce, and the assisted-suicide I Cannot Self-Terminate. See also Staking the Loved One, where this is usually the motive. A subtrope of Shoot the Dog, whose trope namer was a Mercy Kill. A Mercy Kill Arrangement is when a character arranges another character to perform their own Mercy Kill.
As a Death Trope, several if not all spoilers will be unmarked. You have been warned.
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- Big Finish Doctor Who: In Son of the Dragon, Radu performs one of the soldier the Doctor is attempting to save When the Doctor objects, Radu points out that even if the Doctor saves him from blood loss, the wound is poisoned and the man will die a few days hence, and that they have not the resources to care for him in any case.
- On this less permanent level, many sports leagues and organizations have mercy rules in place where a game ends early if one team attains a presumably insurmountable lead after a defined point in the game. Usually, these are recreation and youth leagues, and to a somewhat lesser extent, high school and other sub-professional levels. Examples:
- Baseball and softball: From youth leagues through semi-professional, a common standard for mercy rule is for the winning team to have a 10-run lead after the end of the fifth inning, although some leagues end games before that (usually, the earliest is the third inning) if the winning team has an even larger lead, such as 15 runs after four innings, with the home team, if they are trailing, having a final at-bat. Youth baseball games like Little League have it after the fourth inning, since those games usually last six innings rather than nine. This is a rare aversion to post-secondary teams having a mercy rule in place, as the college level and many semi-pro leagues often have some sort of mercy rule in these sports (although professional does not have this rule).
- Basketball and football: Many state high school athletic associations use a "continuous clock" after a score differential is achieved after halftime. In this instance, the clock doesn't stop for things that it would normally be stopped for, such as going out of bounds or moving the down markers in football, or fouling in basketball, although the clock would stop for such things as time outs. States with continuous clock rules still often have the discretion to end a game early, provided that it is halftime or later.
- Combat sports: technical knock-out, by which the referee can end the fight early if he deems one of the combatants cannot continue without risking permanent damage or worse. The act of throwing the towel and forfeiting a match is based on the same principle, only done by a fighter's cornermen even against his wishes.
- Soccer: Most youth leagues and a number of state high school athletic associations have rules ending games at halftime or later if a certain goal difference is reached; usually, this is the leading team ahead by 10 goals.
- In Anathema, technically every murder is this. Either you murder your victims or they, and everyone else, will suffer a slow, horrible, death as the planet becomes uninhabitable.
- It's a common joke about the legendarily terrible game FATAL that the spell of the same name, which kills absolutely everything in the world, is this. Oh, and it's possible to cast it by accident when you miscast an entirely different spell.
- Mage: The Ascension has the Euthanatos, an entire splat of mages (player character mages, no less) who use this as their hat.
- In the Ravenloft setting, where lycanthropy is even more virulent and dangerous than in most D&D settings, this is the standard and expected outcome if an attempt to cure the affliction fails. As such a failure generally indicates an afflicted person is coming to like their condition, it's as much mercy for everyone else as for them.
- Vampire: The Masquerade has Salubri-antitribu with their special Discipline Valeren. One of the abilities of that Discipline is to give instant and painless death to person, actively willing it.
- The humans of Warhammer40000 refer to mercy kills as "The Emperor's Peace."
- Space Marine Apothecaries carry a special pistol for just this purpose...and then they chainsaw open the chest of the fallen to get at his genetic material. With the utmost respect, of course; without those "gene-seeds", the many chapters of Space Marines wouldn't be able to replace their losses.
- Similarly, but without the chainsaw, psykers carry what they call "Mercy Blades". When the Warp is trying to get into your head, a quick stab through the heart is vastly preferable.
- The Commissar's (and formerly the Sanctioning Officer's) special rule "It's For Your Own Good" evokes this on psykers when their own hands cannot be trusted.
- This is the standard Imperial response to any human infected by Genestealers. Since the infection is incurable by normal Imperial medicine, and causes the infectees and their children to be subordinated to the Tyranid Hive Mind, it's considered better for everyone to end it quickly.
- Exterminatus could be considered this. When a world has been overrun by xenos or chaos forces and is beyond saving, Imperial forces will order the planet's biosphere wiped clean through orbital bombardment, cyclonic torpedoes, virus bombs or other methods of destruction, sparing the inhabitants of the planet from further suffering.
- In Warhammer Fantasy Roleplay, Amethyst wizards have a spell for Mercy Kills. It can be used on any enemy who has taken Critical Damage. This makes it very handy for dispatching that one beastman who has Tzeentch's own luck on the Critical Hit table and goes 3+ rounds without actually taking a Critical Hit that will kill him.
- Older Than Feudalism: Though it tends to get lost due to Values Dissonance, this is one reason Euripides' Medea opts for Offing the Offspring when Jason marries someone else, since as a foreign-born woman her children would lose their Greek citizenship and inheritance once Jason remarried, and were very likely to be sold into slavery.
- In one of the routes in Tsukihime, Akiha is succumbing to her demon blood and begs Shiki to kill her if she loses control. When she does, if you don't keep your promise and keep her alive, you get a depressing "Normal Ending" where she's basically become a mindless, bloodsucking doll under Shiki's care for the rest of her "life". If you do kill her, you get a Bad End. No, to get a decent ending you have to Take a Third Option. Also, Shiki killing Satsuki after she became a murderous vampire, earlier on.
- Fate/stay night:
- In Heaven's Feel route, Shirou is forced to do this to Saber after she is hit by The Corruption and turned into Saber Alter. Especially bad, as Saber had been a symbol of purity throughout the game, and had been your primary love interest in the first route, as well as a possible love interest in the second route.
- In one bad ending Shirou is reduced to a torso and head suspended in a liquid that has left him drugged into insensibility. When Rin finds him she shatters the container, letting Shirou die.
- Kerkan from Sekien no Inganock is a serial killer who believes everyone is eligible for a mercy kill, but tends to only pick the truly hopeless.
- New Danganronpa V 3: Gonta is tricked into killing Miu when Kokichi tells him that, if everyone guessed wrong, it would end out in a mercy kill with the students (Kokichi included, but Gonta and the mastermind excluded) on the receiving end.
- While all spirits in Spirit Hunter: NG are essentially being put to rest rather than forced to continue their tumultuous existence, this is explicitly the case with the Screaming Author, who outright asks Akira to set it on fire so that she can escape the monstrosity that her body has been turned into.