A character has a major problem. It may be a Superpowered Evil Side, reliance on a Dangerous Forbidden Technique, or some form of The Virus or The Corruption (vampirism, lycanthropy, or they're a Zombie Infectee). Or it may simply be that they suffer from a terminal illness which may at some time take away their independence and make their life a living hell. Either way, this person has something dark within them, an essence which poses a significant existential threat to them and probably to their loved ones as well.
In order to ease their anxiety about this danger, the person may ask a difficult favor of another: to end their life if things reach a point where safety and quality of life can no longer be assured. This is obviously a very painful thing to have to do, and asking it requires a great deal of trust, both in one's emotional composure in ending another's (often a loved one's) life, often in cold blood, as well as the physical ability to do so, especially if the victim is overcome by a supernatural force and is actively attacking them. The person may sometimes give the other a special weapon of some kind that will be effective against them.
The conversation by itself can have significance in a plot, by showing the amount of concern one character has for the threat that exists to themselves or to others, and the faith they have in another character to help relieve this problem. Often, however, such a scene sets up a Chekhov's Gun whereby the feared transformation or disease does occur and the promise may have to be carried out. Often the recipient of the promise will verbally remind the other and ask to be killed before it's too late, while the other hesitates and hopes that matters can be restored to a healthy balance. Whether they ultimately must go through with the deed depends on the nature of the threat and the optimism of the story; if they do, it's a Mercy Kill with possible overlap of I Cannot Self-Terminate. Note that this trope only applies if the discussion occurs well before the actual transformation or threat manifests.
Of course, the mundane version of this is common in real life, as many families struggle to find the best way of dealing with a disease that can destroy a person's quality of life.
A sub-trope of The Promise and Thanatos Gambit. A super-trope to Kryptonite Ring, when a character entrusts a special weapon to an ally in case they go bad, and Suicide by Assassin, when a character hires a contract killer as part of an assisted suicide. Compare to Better to Die than Be Killed, in which a character tries to commit suicide rather than face an undesirable fate. Also compare Suicide Pact when individuals arrange to commit suicide together or kill each other under some circumstances. See also Betrayal Insurance, which is less about mercy and more about preventing other characters from stepping out of line.
- Blood+: Saya made Haji promise to kill her once they finally succeed at killing Diva, in order to end the chiropteran threat once and for all. Ultimately, however, Haji and Kai convince her not to go through with it and go on living.
- The eponymous super soldiers of Claymore have an understanding that if the demonic power they rely on gets out of control, one of their compatriots must kill them immediately, lest they become an Awakened Being, a threat on a far greater level than the monsters they usually fight.
- Fullmetal Alchemist: Colonel Roy Mustang aims to be a good leader for the country of Amestris. He asks his trusted subordinate Riza Hawkeye to kill him if he ever "goes too far", as he fears that his ambitions might lead him to do too much unsavory things.
- Played With in Noir, where Mireille promises to kill Kirika after the two of them discover the truth about their past connection — Kirika is not particularly keen on dying, but doesn't mind it, either, since she doesn't have anything to live for, anyway. Much later in the series, after learning the truth that Kirika killed Mireille's parents to prove she is worthy of the True Noir title to Les Soldats, who are now coming to take her back, Kirika tearfully begs Mireille to fulfill her promise, but Mireille, having fallen deeply in love with her through the tribulations they faced together since her promise, cannot bring herself to pull the trigger.
- In School-Live!, Yuuri promised Kurumi that if she became a Zombie Infectee that Yuuri would be the one to kill her before she turned. Early in the manga (and late in the anime), Yuuri becomes dangerously close to having to kill Kurumi as she slowly succumbs to a zombie bite. Thankfully, Miki arrives with an experimental vaccine that saves Kurumi... for the time being.
- In Street Fighter Alpha: The Animation, Ryu at one point implicitly enlists Ken to stop him should Ryu completely fall to the Satsui no Hado, and even have a match to make sure Ken is still capable of keeping up with him if need be.
- In Avatar: The Last Airbender The Promise, Zuko asks Aang to kill him if he ever shows signs of turning out like his father.
- Civil War II reveals that Bruce Banner made such an arrangement with Hawkeye, wherein the archer would kill Bruce before he could transform into the Hulk. Hawkeye could see a flash of gamma green in Banner's eye, prompting him to fire a special arrow.
- Played for Laughs in the Belgian comic Le Chat, which has one strip where the titular cat fills almost an entire page with abominable cat puns, waking up in the penultimate panel.
The Cat's Wife: What is it, dear?
The Cat: I had a horrible nightmare where I took the path of cheap laughs!
The Cat's Wife: Oh don't be silly, you know I'd have you put to sleep before that could ever happen.
The Cat: You promise?
- In Runaways, Nico asks Gert to kill her if she ever lost control of the Staff of One. Gert has adamantly refused any such arrangement.
- Word of God has confirmed that the Sin City short story "The Customer Is Always Right" is an instance of this trope. The unnamed female protagonist had previously been in a relationship with a gangster who threatened her with a particularly brutal revenge when she broke up with him, so she hires an assassin called "The Salesman" to kill her so she could be sure of dying on her own terms.
- In Star Wars: Legacy, the Fel Emperor basically has an agreement with his Imperial Knight bodyguards; if he falls to the Dark Side and can't be redeemed, they must kill him.
- Weight of the World: More like a "Mercy Coma Arrangement" since personifications cannot die and instead go into a brief coma-like state when they are badly injured. America asks England to put a nigh-unbreakable sleeping curse on him that can be activated if he is about to be captured by Salem. As long as he is comatose, Salem cannot use him to get the Relic of Choice and Remnant and Earth will be safe. The request is treated like he's asking for a mercy kill in the face of capture. England agonizes over the decision and is horrified that his brother would ask him to "kill" him. After realizing he will condemn his brother to torture if he doesn't do it, he agrees.
- Aliens. Ripley and Corporal Hicks are faced with the threat of being impregnated by alien facehuggers and used as incubators to hatch new aliens. Ripley asks Corporal Hicks to kill her if this happens to her, and he agrees emphatically.
Ripley: Hicks, I'm not gonna end up like those others. You'll take care of it, won't you?
Hicks: If it comes to that, I'll do us both.
- Avengers: Infinity War: Gamora has Star-Lord promise to kill her if Thanos ever captures her. When Thanos does grab her, Star-Lord tries to keep his promise without wasting a second, but Thanos stops him using the Reality stone.
- Early in Serenity when the crew is pursued by Reavers, (Ax-Crazy cannibals who, if you're not lucky, may not wait until after they kill you to eat you) and it looks like they're on the edge of capturing Jayne, Jayne asks Mal to Mercy Kill him if the Reavers capture him. When Mal draws his gun and appears to immediately take aim at Jayne, Jayne hurriedly clarifies that he wants the crew to at least try to save him first.
Jayne Cobb: I won't get 'et! You shoot me if they take me! [Mal draws his gun and appear to aim at the still untaken Jayne] Well don't shoot me first!
- In Starship Troopers, Major Rasczak shoots a wounded soldier and makes clear to his unit that he expects them to do the same for him. When a bug rips off both of Rasczaks legs, he orders Rico to kill him. Rico very reluctantly complies.
- Van Helsing: The title character has been bitten by a werewolf and must use the supernatural strength bestowed by the transformation to kill Dracula before the twelfth stroke of Midnight. While Van Helsing deals with Dracula, he sends Anna and Carl to search Dracula's castle for a cure for lycanthropy, and gives Carl a silver stake, insisting that Carl must destroy him if they fail to administer the cure before the transformation becomes permanent.
Van Helsing: If I'm not cured before the twelfth stroke of midnight... *brandishes stake*
Carl: I'm not sure if I could.
Van Helsing: You must.
- Animorphs: In book 26, The Attack, Jake manages to acquire the DNA of one of Crayak's Howlers. As they're seemingly just Ax-Crazy murderers who kill entire races For the Evulz, when Jake is about to morph into it, he has Rachel and Marco on standby, ordering them to kill him should he lose control of the Howler's instincts. They don't have to do so, because as it turns out, the Howlers in fact have the personalities of young children, consider their battles to be fun and exciting games, and have no idea that they're hurting real people.
- At the end of the Discworld novel The Fifth Elephant this is Implied. Angua (a werewolf) asks her boyfriend Carrot if he would kill her if she went off the rails the same way her brother did (having gone into a half crazed state after his Evil Plan was thwarted). Carrot answers "Yes", which was exactly what she wanted to hear.
- In The Dresden Files, Harry is in urgent need of power and can only get it by pledging to become the Winter Knight of the Fae. Knowing what a total monster the last man in that position became, he arranges to be killed as soon as he accomplishes his goal and has his memory of the assassination order removed. However, his new boss is the kind of entity who doesn't let someone off the hook just for being shot through the heart...
- Dumbledore in Harry Potter secretly arranges a mercy kill for himself with Severus Snape when he learns that Lord Voldemort has set Draco Malfoy the task of killing him. He's dying anyway as a result of mishandling a cursed artefact, but doesn't want the boy's soul to be tainted by murder, and is afraid of a much less dignified death in the event that one of the more sadistic Death Eaters gets to him first. In addition, he had hoped that the arrangement would break the power of the Elder Wand, its last owner dying undefeated.
- In the fifth book of The Helmsman series, the protagonist is stranded on an enemy planet with three other men, one of whom is the Emperor. The Emperor cannot afford being taken captive, so he makes the other three swear they'll shoot him first.
- The Hunger Games: Before Gale and Katniss go to infiltrate the inside of the Capitol, they promise each other that if one of them is captured, they other one would shoot them rather than let them get taken. However, when Gale gets carried off, Katniss can't bring herself to kill him in time.
- In the fourth novel of the Hyperion Cantos, after de Soya's ship is destroyed, the last of his crew are stranded without resurrection equipment. Since that means the Captain will suffer a serious case of Came Back Wrong once he succumbs to his wounds, he makes one of the survivors promise he'll vaporise him first.
- Dekka in Gone, fearing that she is infected by the parasitic bugs that she was trying to kill, makes Sam promise to kill her if it comes to that. Sam breaks his promise, keeping her alive to do "surgery" on her - physically tearing the bugs out of her body and then having Lana heal the resulting wound.
- Agents of S.H.I.E.L.D.:
- Coulson asks May to do this if GH-325 starts to drive him as mad as it did John Garret.
- In the fourth season, it turns out Senator Nadeen has an agreement with her brother that should one of them turn Inhuman, the other will shoot him/her.
- Angel: When Angelus is intentionally summoned, via removal of Angel's soul, to get information about the Beast, Angel makes his team promise to kill him if Angelus gets out of his cage. He does cause more trouble than they thought, but they don't have to do it.
- On House, Thirteen performs a mercy kill for her brother, who was suffering from Huntingtons disease, an incurable degenerative disease their mother suffered from and that Thirteen has as well. House offers to do the same for Thirteen when the time comes, though presumably this is forestalled by House's faked death.
- Jessica Jones (2015): When Jess and Simpson are getting ready to try to capture the Psychopathic Manchild Kilgrave, they both tell the other to take them down if Kilgrave manages to control them with his Compelling Voice.
- In season 2, Sam makes Dean promise to kill him if Sam starts to become evil due to the demon blood in his veins.
- In season 2, Madison asks to be killed once she finds out she's a werewolf and all cures have failed. Sam and Dean grapple with the idea of killing her while she looks human and suggest that she can be locked up during the full moon, but she points that she's already escaped more than once and killed people. In the end, Sam ends up shooting and killing her with a silver bullet.
- In season 6, Lenore, a previously "vegetarian" vampire, begs Sam and Dean to kill her because her connection to Eve, the mother of all monsters, is pushing her to drink from humans. Sam and Dean are hesitant because they believe Lenore to be a "good" monster and they suggest detox. Castiel, however, has no such compunctions and smites her.
- In season 10, Dean makes Castiel promise to kill him if the Mark of Cain causes him to lose control and begin murdering people.
- Also in season 10, Cain makes Dean promise to kill him so that he doesn't have to live in a murderous rage caused by the Mark of Cain. Unlike the previous two examples, Dean is forced to actually make good on the promise when, triggered into a blood rage when he is ambushed and has to slaughter dozens of demons to escape, Cain sets on a systematic campaign to murder all of his descendants, which Castiel notes numbers in the billions.
- From Jekyll & Hyde we have Dr. Jekyll asking his friend Utterson to kill him should Jekyll's evil alter ego Mr. Hyde take control at an inconvenient time. Depending on the show, Utterson either shoots Jekyll at Jekyll's wedding after Hyde shows up and starts killing people, or Jekyll takes control one last time to run into Utterson's sword after Utterson finds himself unable to do it.
- Gebel and Miriam from Bloodstained: Ritual of the Night are both extremely powerful "Shard-Binders" - as a result of experiments carried out on them by the alchemists guild, they can transform demon corpses into crystalline shards and use them to cast magic. Because they were experimented on without their consent, both maintain that this power is only as evil as the Shard-Binder's intentions; but just in case, they've sworn a pact that if either of them ever appears to be acting without free will, the other must hunt them down and destroy them.
- In Diablo III, optional dialogues with the sidekick Kormac reveals that he was initiated into the Templar order by way of torture to remove his memories of a supposed sinful past. The player character suggests that he should try to recover his past, and he reluctantly agrees, on the condition that the player character will kill him if he relapses into evil. Their reactions to this request vary, but all characters agree to the terms. As it happens, the fear is moot, as Kormac's sins were invented by the order to make him a willing soldier.
- The Elder Scrolls IV: Oblivion: The Shivering Isles expansion pack has Hirrus Clutumnus, a man who is cursed with suicidal depression but too afraid to take his own life in case his soul becomes trapped on the Hill of Suicides. To get around this, he gives you a quest to kill him in a manner that he's not going to see coming, and in return he gives you permission to loot a key from his body after his death, which unlocks his house and a strongbox containing his most valuable possessions.
- In the backstory for Fire Emblem: Path of Radiance, Greil became a killing berserker while under the influence of Lehran's Medallion. His wife Elena managed to take the medallion away from him, but took a fatal blow when Greil ran his sword into her. Horrified at what he did, Greil crippled his sword arm and hired the assassin Volke for various things, one of which is to be Greil's killer if the former ever went berserk again. Of course, that never came to be, as Greil was killed by the Black Knight. Volke kept his arrangement with Greil a secret, only telling Greil's son Ike when the latter proved mature enough to know.
- Honkai Impact 3rd: In one of the supplementary comics, it's explained that Valkyries who fight in the frontline against the forces of Honkai will always be under the threat of getting Honkai corruption and turn into a mindless, murderous zombie. It's a common arrangement that, if a Valkyrie ends up getting corrupted by the Honkai, their friend would have to kill them on the spot.
- Kingdom Hearts: Birth by Sleep: After learning that he is some kind of weapon and is destined to fight Vanitas in order to forge it, Ventus asks Terra and Aqua to put him down the moment he does.
- In the ending of Megaman X 4, after seeing the chaos caused by Repliforce's coup and how easily Sigma was able to trick them into becoming his pawns, Megaman X asks his friend Zero to destroy him if he ever shows signs of going Maverick.
- Left 4 Dead: The four playable survivors are immune to the virus that turns people into zombies. However, Bill tells the other three to shoot him if he ever starts to turn.
Francis: What if your beard starts to turn? Can I shoot that?
Bill: You are an idiot.
- In Neverwinter Nights 2: Mask of the Betrayer, your character is afflicted with a curse called the Spirit Eater, which requires them to feed on the spirits of others and eventually drives everyone who suffers from it insane. If you recruit Okku the Bear God as an ally, you can gain influence with him by giving him permission to rip out your throat if you ever lose control of the curse, and he promises to do so, and to not take any enjoyment from it.
- In Suikoden V: Ferid orders Georg to kill Arshtat in case she becomes insane by the power of the Sun Rune.
- Near the end of Castlevania: Aria of Sorrow, Soma asks this of Julius Belmont, in the event that Soma, who is Dracula's reincarnation, fully becomes Dracula. The Bad Ending of Aria and its sequel Dawn of Sorrow both show Julius making good on his promise.
- In Divinity: Original Sin II: Lohse spends most of the game trying not to succumb to Demonic Possession and can only be recruited as a companion after Act 1 if the player character agrees to kill her if she needs it.
Lohse: It's out for pain. Domination. Total domination. I was wrong to think I could hold it off. I might hurt you. I might hurt anyone. Could you stop me? Really stop me, if it came to that?
- Living wills are a legal arrangement people make which may provide consent for the removal of life support (and thus death) in the event of falling into a permanent coma.
- Assisted suicide, which is legal in certain jurisdictions such as the Netherlands or the U.S. State of Oregon falls into this. A terminally-ill person (who is able to give informed consent) makes a contract with a doctor to have the doctor inject them with lethal drugs to stave off the suffering the ill person would otherwise endure.
- Samurai committing seppuku would have a second standing by to decapitate them, to spare themselves and the audience the pain/spectacle of a slowly dying man spraying blood and organs everywhere.