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[[quoteright:350:[[VideoGame/Left4Dead http://static.tvtropes.org/pmwiki/pub/images/L4D_mercy_kill_5_4096.jpg]]]]

->''"Will it be no joy to think of hereafter in the silence of the night when sleep is not, 'It was my hand that sent her to the stars. It was the hand of him that loved her best, the hand that of all she would herself have chosen, had it been to her to choose?'"''
-->-- '''Van Helsing''', ''Literature/{{Dracula}}''

One of the occupational hazards of DemonSlaying involves the death of your friends and family out of vengeance from the monsters you hunt. But sometimes the monsters don't just kill your loved ones. No, sometimes they [[TragicMonster turn them into one of their own]]. This puts the protagonist in the awful position of having to kill someone they truly loved.

Expect this trope to crop up in VampireFiction, almost always when the monsters are AlwaysChaoticEvil. Usually averted with the FriendlyNeighborhoodVampire. This situation can also crop up with other monsters, the most common variations being [[OurWerewolvesAreDifferent werewolves]] and [[ZombieApocalypse zombies]].

See also AndThenJohnWasAZombie. May be a case of VanHelsingHateCrimes when the monsters are ''not'' AlwaysChaoticEvil. Related to TranshumanTreachery, as this is what the newly turned are feared to commit. Often the only "good" way WhatHappenedToMommy can end up. Frequently a TearJerker.

See also ICannotSelfTerminate, MercyKill. SubTrope of KillTheOnesYouLove and TragicMonster. Not to be confused with ''[[StalkerWithACrush stalking]]'' [[StalkingIsLove the loved one]].

----
!!Examples:

[[foldercontrol]]

[[folder:Anime and Manga]]
* ''Anime/MagicalGirlLyricalNanohaAs'' ends with everyone defeating the [[EnemyWithout Self-Defense Program]] of Reinforce, the former Book of Darkness. She's grateful, but the next day informs them that they have to finish the job with her too or else her programming will be forced to regenerate the [=SDP=]. Nanoha and Fate do, because it would be too cruel to make Hayate do so.
* ''Anime/PuellaMagiMadokaMagica'':
** [[spoiler:Kyouko is forced to kill Sayaka, who turned into a Witch]].
** [[GroundhogDayLoop In the previous iteration]], it's Homura who must MercyKill Madoka. [[CynicismCatalyst This pushes her into]] {{Antihero}}dom.
** Darkly {{Invoked}} too by [[spoiler:Mami in the previous iteration, rather than seeing her other mentee go Sayaka's path, she would rather kill them. Madoka stopped her from killing Homura... by shooting her first]].
* In ''{{Hellsing}}'', [[LadyOfWar Integra]] is instructed to put down her subordinates-turned-ghouls herself, since she was their leader and thus their being defeated and transformed is her responsibility.
* In an inversion, ''RosarioToVampire'' has the vampiric Moka nearly beating [[spoiler: the human Tsukune]] to death because he had become a ghoul that was dangerous to everyone around him. She considered it her responsibility to finish him because she was the one who caused the transformation.
* ''LightNovel/FateZero'' shows us the consequences of ''not'' doing it: after drinking a potion that transformed her in a vampire, Kiritsugu's childhood friend [[UnwittingInstigatorOfDoom Shirley]] begged Kiritsugu to kill her while she was still capable of resisting her instincts, but when Kiritsugu couldn't go through it (opting instead to ask the local priest for counsel) she caused a [[ZombieApocalypse vampire outbreak]] [[DoomedHometown that claimed everyone on the island but Kiritsugu and his father]]. The experience, including the Church and the Mage Association killing all the vampires and burning down the place and having to kill his own father for experimenting on vampirism, was Kiritsugu's CynicismCatalyst: since then Kiritsugu never hesitated to pull the trigger whenever necessary, and when he got to choose between shooting down a plane with [[ParentalSubstitute his mother surrogate]] and a horde of ghouls on it and risking it to cause another vampire outbreak he immediately shot down the plane.
[[/folder]]

[[folder:Comic Books]]
* Cassie Hack in ''ComicBook/HackSlash'' had to do this to her [[DeathByOriginStory deranged undead mother]]. It ended up as a recurring FlashbackNightmare for her. She's actually done it ''three times'' so far. Delilah Hack just doesn't seem to want to stay dead.
* The last act of Dmitri Mishkin, in ''ComicBook/HouseOfMystery'''s "I... Vampire" series, was to stake his own mother. Kind of an odd encounter, given that she'd been turned when Dmitri was a small boy and he'd been hunting her for decades, so his mom looks a good fifty years younger than he does.
* In Marvel's ''Comicbook/TheTombOfDracula'', Drac is clockwork predictable with this trope. Virtually no one outside the 'Bridge Crew' was taken prisoner without being turned.
[[/folder]]

[[folder:Fan Fic]]
* Done with a twist in ''FanFic/TheReturn'' when one character is forced to shoot her sister who has been turned into a demon, so that she can be given an EmergencyTransformation to save her.
* In Creator/StephenKing's ''NightOfTheLivingDead'' fanfic story "The Reach", the inhabitants of a small New England island off the coast of Massachusetts cooperate to rid their island of zombies and to prevent the local graveyard from spawning new ones. When the strain gets to be too much for one old man, he begins suffering a heart attack. Rather than risk dying and coming back as a zombie, he tells his sons that he's going to recite the Lord's prayer, and when he says "Amen", they are to put the muzzles of their shotguns up to his chest and pull the trigger, just so he can spare them this.
[[/folder]]

[[folder:Film]]
* ''Film/IAmLegend'' featured [[spoiler:the main character's dog being bitten by the "vampires" and he must put her down when it becomes obvious that the antidote he's been trying to develop didn't work. Worse than a lot of examples because the main character is the last human left in a city full of monsters, and the dog was his only companion.]]
* ''Film/FromDuskTillDawn''. Seth has to kill his brother. Kate has to kill her father ''and'' brother after her brother found himself unable to kill the father and was infected as a result.
* Parodied in ''Film/DraculaDeadAndLovingIt'', where Van Helsing (played by Creator/MelBrooks) invokes this trope as an excuse to not have to stake Lucy:
-->'''Van Helsing''': It must be done by one who loved her in life!
-->'''Jonathan Harker''': But I only liked her!
-->'''Van Helsing''': Close enough!
* Subverted in the first ''Film/CountYorga'' film, one of the protagonist is given the chance to stake a female friend of his that been turned by the title character (she actually stands there and waits on him). He considers it but ultimately can't do it and leaves her in her undead state.
* In ''Film/ShaunOfTheDead,'' Shaun has to kill [[spoiler:his zombified mother]]. Subverted in the case of his [[spoiler:zombified best friend; Shaun locks him in a shed in his back yard and they play video games together.]]
* ''Film/{{Blade}}'' has to stake [[spoiler:his own mother]] in the first film; [[spoiler:he believed she had died giving birth to him after a Vampire bite]].
** He also [[spoiler:gives Whistler a gun to shoot himself before he fully turns. Blade does an UnflinchingWalk as a gunshot is heard behind him. Subverted when it turns out that Whistler missed and survived]].
* In ''Film/NightOfTheLivingDead'' (1990), Barbara is forced to shoot her friend Ben after he dies and turns into a zombie.
* ''Film/ResidentEvil'' (2002). Matthew Addison has to kill his ally Rain Ocampo after she turns into a zombie.
* Non-fantastic example in ''TheSearchers'': Debbie Edwards's own family expect her uncle to kill her after they learn she's been indoctrinated by the Comanches who kidnapped her into becoming their squaw - and they have hardly any regrets, because after all Debbie is now the enemy. Of course, since said uncle is a fanatical Indian-hating psychopath, Debbie's blood tie to him never even enters the picture [[spoiler: until he actually has the opportunity to do it, and he experiences an AdoptTheDog moment]].
* In ''Film/{{Zombieland}}'', Columbus and Tallahassee come across Wichita and her zombie-bitten sister, Little Rock. They hand her the gun to do the mercy kill, but things don't go ''quite'' as they plan because [[spoiler: they're faking the zombie bite just so they can get a hold of Columbus and Tallahassee's guns and truck]].
* In ''Film/TwentyEightDaysLater'' Selena tells Jim right near the start that if he got infected she'd kill him "in a heartbeat", and she proves this by killing one of her other friends when he turns. At the climax she believes Jim has been infected due to the [[EyeScream shockingly brutal]] killing the formerly gentle man commits. [[spoiler: She can't bring herself to actually attack him.]]
** Earlier on [[spoiler: Frank]] is infected but Jim can't bring himself to deliver the death blow, even though he knows he's only got seconds before they're all in danger and Selena is screaming at him to do it.
* When Secker and Paxton in ''Film/TasteTheBloodOfDracula'' find vampirized Lucy in a casket, Secker tries to stake her, only for Paxton to chase him away, so that the task of slaying his daughter would befall on him alone. He fails because he takes too much time to do the deed.
[[/folder]]

[[folder:Literature]]
* Best known example from Bram Stoker's ''Literature/{{Dracula}}'' when Helsing shows the men first hand of Lucy's undead form after she "dies" and seeing she's more monster then human. He forces her fiancee Arthur to do it claiming a loved one should set her soul free.
* In ''Literature/IAmLegend'', Neville must kill his reanimated wife when she returns.
* ''Literature/TheGardellaVampireChronicles''.
* Subversion in ''[[DenOfShadows All Just Glass]]''. [[spoiler:The main character herself is a vampire hunter and witch who is turned into a vampire. Her sister, another hunter, wants to do this, but ultimately can't bring herself to do it.]]
** There's another subversion in the book that comes right before it in the series, ''Shattered Mirror''. [[spoiler: Sarah goes out to kill Kristopher, and then gets turned into a vampire.]]
* In one AnitaBlake book, the parents of a dead girl ask Anita to do this for them before their daughter rises as a vampire.
* Happens to several characters in ''Literature/SalemsLot.''
* In the MercyThompson novels, doing this to werewolves who can't control their "wolf side" is usually the duty of their closest werewolf relatives, but if they can't do it, the Marrok (Alpha-in-chief of all North America's werewolf packs) or his son Charles have to step in. The novels make it clear that both the Marrok and Charles have already done it too many times for their own peace of mind.
* In ''[[Literature/VampireAcademy Blood Promise]]'', Rose has to do this to [[spoiler: Dimitri]].
* In Literature/{{Baltimore}}, or the Steadfast Tin Soldier and the Vampire, Baltimore has to do this [[spoiler: not only to his wife, but his father, mother, and sister as well.]]
* Invoked in-universe in ''Literature/DreamPark'', when the casualties of the South Seas Treasure Game are sent back in make-up to "attack" their former comrades as part of a zombie horde. Gwen, acting under the Game Masters' orders, winds up in a duel with her out-of-game boyfriend Oliver, and he's so shocked that she has to whisper "Kill me, Ollie, please" before he'll play out the scene properly and strike her down with his virtual weapon.
* In the ''Literature/{{Discworld}}'' novels, Angua has specifically ''requested'' that Carrot do this if she ever ends up becoming as monstrous as her brother Wolfgang.
* In ''Literature/TheDresdenFiles'', "Literature/{{Changes}}", [[spoiler: Harry is forced to kill his vampire girlfriend Susan when she turns fully. The situation was further complicated in that the Red Court vampires that sired Susan were about to kill their daughter to activate a spell that would kill her family line, and by killing Susan, Harry could use the spell against the villains, destroying all members of the Red Court.]]
** Made even worse when one realizes that [[spoiler:our hero]] knowingly manipulated the situation so that [[spoiler:Susan would lose control and kill, thereby turning into the youngest Red Court vampire. He hates himself for doing it and will never forgive himself, even though it saved their daughter.]]
** Note finally that [[spoiler:Susan had]] ''regained'' control in the final moments, and knew exactly what [[spoiler:he was doing and why. The fact that she realized it was the only way to save their child, and didn't struggle, arguably made it even harder for Harry to cope with this trope.]]
* Buffy having to stake the vamped Giles in the ''Series/BuffyTheVampireSlayer'' novel ''The Lost Slayer''.
* In the ''MagicTheGathering'' novel ''Nemesis'', Eladamri kills the Phyrexian Belbe, who was created from the corpse of Eladamri's daughter. Overlaps with VanHelsingHateCrime in that Belbe had already gone through a HeelFaceTurn and was actively sabotaging the Phyrexians' plans. NiceJobBreakingItHero.
* In the post-ZombieApocalypse ''Literature/{{Newsflesh}}'' world, it's accepted that if someone is about to amplify (become a zombie), even close relatives are expected to put the victim down for good, both as [[MercyKill an act of compassion]] and to keep the victim from spreading the problem by attacking those around him/her. In the first book, we hear about a mother forced to kill her child and see an about-to-be-zombie shot by a close friend. Then there's [[spoiler:Shaun performing a MercyKill on his [[BrotherSisterIncest lover]]/[[NotBloodSiblings adoptive sister]] Georgia]], which sends him into SanitySlippage for the next two books.
[[/folder]]

[[folder:Live Action TV]]
* ''Series/BuffyTheVampireSlayer'', many times.
** Most notably when Buffy had to kill Angel to stop Acathla.
** The first episode also featured Xander staking one of his best friends - by ''accident.'' He couldn't do it, and only survived because a panicked bystander unknowingly shoved the vamp onto his raised stake.
** In "Phases" Angelus turns a schoolmate of Buffy's into a vampire, knowing that [[BreakTheCutie she'll be forced to stake her]] when she goes to examine the corpse. A flashback episode in ''Series/{{Angel}}'' showed him pulling the same stunt with Holtz's daughter.
** Spike had to stake his newly-sired mother after it was made clear that she was a completely different person. And evil. And trying to shag him.
** Season Two is a protracted inner struggle for Buffy, who can't bring herself to stake Angelus in their first encounter.
* On ''Series/{{Angel}}'' Gunn staked his sister and Holtz was forced to throw his daughter into the sunlight.
** Subverted with Illyria and Spike: he didn't care if she had Fred's body, even though Fred was his buddy.
-->'''Illyria''': You held affection for this shell, and yet you strike at its form without remorse.
** None of them really showed any hesitation over trying to kill Illyria. Even Wesley tried, though Illyria's resemblance to Fred got to him later. They were just totally incapable of achieving it.
** A RunningGag is that whenever a member of Team Angel warns a co-worker to be ready to kill them if they turn evil, they agree a lot more readily than comfortable. Especially Connor.
** On two occasions a cast member had to dismember the body of a loved one in the mistaken belief they had been killed by vampires.
** Subverted in "Double or Nothing". Angel is playing an AbsurdlyHighStakesGame that if he loses will cost him his soul (which will turn him evil). He hands Cordy a wooden stake and says she knows what to do if that happens. When Angel does lose the game, Cordy instead rams the stake through the BigBad's palm, holding him in place so Angel can lop off his head.
* Sort of done in ''Series/BeingHuman''. [[spoiler: Lauren begged Mitchell to stake her and he did. Only counts as a "sort of" example as Mitchell was the one who turned her into a vampire in the first place.]]
** Done totally straight by [[spoiler: George to Mitchell. He even makes sure to tell him "I'm doing this because i love you."]]
** Now in the US version, Aidan is forced to stake [[spoiler: Bernie, his ten year old neighbor, because as a child he cannot control his emotions or appetites]]. A little later on, Aidan again stakes [[spoiler: Rebecca at her request.]]
* Javier did this to his girlfriend on ''Series/BloodTies''.
* The ''failure'' to do this quickly enough, when his infected friend hands him a silver-loaded pistol and begs him to end his suffering, is what set Eric Cord on his quest in ''{{Werewolf}}''.
* On ''Series/{{Supernatural}}'', the second season episode "Heart" forced Sam to do this for GirlOfTheWeek Madison, who had become a werewolf.
** Gordon beheaded his sister after she became a vampire, a fact which disturbs Sam and Dean. Part of this was probably because he tracked her down and killed her (in an episode showing that FriendlyNeighborhoodVampires, or at least non-murderous ones, existed too) instead of her attacking him.
** Invoked by Meg when she possessed Sam and tried to convince Dean that his PsychicPowers had turned him evil, knowing that killing his brother would cause him the most pain and that what he tried wouldn't kill her, just Sam's body.
** When he's suspected to have the [[TheVirus Croatoan Virus]], Dean [[{{Defied}} refuses to]] kill him or let anyone else do it, instead deciding to stay with Sam until he "dies".
** Bobby had to do this to his wife twice: when became [[DemonicPossession possessed]], he stabbed her, not knowing what else to do, and later kills her again when she comes back to life as a zombie.
-->'''Bobby:''' She was the love of my live. [[TearJerker How many times do I have to kill her?]]
* Krista in ''BladeTheSeries'' does it to [[spoiler:her mother]] after her plan to turn [[spoiler:her]] into a vampire goes awry.
** Subverted in the case of [[spoiler:Krista's uncle]] after he is [[spoiler:bitten by Krista's newly-turned mother]]. Blade [[OffWithHisHead beheads]] him as a precautionary measure.
* A vampire hunter in a ''{{Sliders}}'' episode does this after finding his wife in a coffin, before the other vampires swarm him. Quinn is ready to do this to Wade, except she hasn't been bitten and was just sleeping in the coffin.
* On ''Series/ForeverKnight'', [=LaCroix=] has to stake his daughter for much the same reason Spike staked his mom on Buffy.[[note]] Wanting to 'shag' him.[[/note]]
* Many times on ''Series/TheWalkingDead'', given the fact that it's a show about the zombie apocalypse. A few notable examples: Andrea held her sister Amy's body until she rose again as a walker before stabbing her in the head. (Stabbing walkers in the head is the only way to stop brain activity and kill them.) In season 2, Hershel is unable to kill his zombified family members and instead keeps them in a barn. Shane goes and kills them all anyway.
** After the season 2 reveal that [[spoiler: the virus is in all of them and when they die, regardless of if they were bit or not, they will rise again as a walker]], it becomes standard procedure to stab their loved ones in the head whenever they die. One of the more disturbing examples is [[spoiler: Carl, a young teenage boy, stabbing his mother in the head after she dies in childbirth]].
[[/folder]]

[[folder:Music]]
* In the Music/SoundHorizon song "Koibito wo Uchiotoshita Hi" ("The Day I Shot Down My Lover"), a girl must kill her lover - who saved her life from a monster - when the venom from his battle wound cause him to turn into a monster himself.
[[/folder]]

[[folder:Tabletop Games]]
* In the ''{{Ravenloft}}'' Dr. Rudolph Van Richten dedicated his life to [[TheHunter hunting vampires]] after his son fell victim to one. But while he slew the monster that was responsible, he was forced to MercyKill his own son first - who had been transformed into an undead creature himself.
[[/folder]]

[[folder:Video Games]]
* [[spoiler:Saber]] in Heavens Feel route of ''VisualNovel/FateStayNight'', though in this route she's not a love interest so much as a very close friend. Quite a downer when the Taiga Dojo for ''not'' doing points out if you want to save her you can go play another route, because it's not happening here (assuming the resultant horrifically Bad End didn't floor you already).
* [[spoiler:Akiha]] makes you promise this in ''{{Tsukihime}}'' when her demon blood is taking over. [[spoiler:Averted in the True End, though, when you TakeAThirdOption by returning your life force to her. The Normal End is actually ''worse'' than if you had killed her.]] Also barely averted in Kohaku's route with [[spoiler:Akiha again.]] Not [[DemotedToExtra Satsuki]] though. She was just a classmate, so Shiki is just upset he had to kill someone at all.
* In ''VideoGame/MeltyBlood'', Riesbyfe's path takes place in a timeline where Sion fell to vampirism, and she had to kill her. Then she had to do the same thing to Dust of Osirus; a clone of Sion's from the future. She does this knowing that Dust of Osirus is sustaining Riesbyfe's existance. "I hope to see you again, my friend; after the darkness."
* In ''[[VideoGame/PrinceOfPersiaTheSandsOfTime Prince of Persia: The Sands Of Time]]'', the Prince finds himself having to fight his father as a Sand Demon.
* In ''VideoGame/FinalFantasyTactics'', when Ramza has foiled the Lucavi's plots once too many times, Folmarv abducts his older half-brother, Zalbaag, and turns him into an undead monster. Although Zalbaag is perfectly [[AndIMustScream conscious and aware of his condition]], his body will relentlessly pursue Ramza and try to kill him, at which point he begs the younger Beoulve to put him out of his misery. Granted, Zalbaag was [[AristocratsAreEvil a bit of a jerkass]] throughout the game, but he was on the path of [[RedemptionEqualsDeath redemption]] and actually cared for his family, making this a TearJerker.
* In ''VideoGame/CrisisCore: VideoGame/FinalFantasyVII'', [[spoiler: Zack has to kill his mentor, Angeal, who actually makes himself monstrous to make it easier for Zack to kill him.]]
* In ''BaldursGate II'', [[spoiler: your love interest (if you have one)]] gets turned into a vampire and you have to kill them. [[spoiler: There's a way to bring them back to life though.]]
** And if you didn't, ''don't'' spend ages trying to figure out what you're supposed to do with Bodhi's heart.
* Pretty heart wrenching in ''VideoGame/ParasiteEve'' when the police dog, Sheeva, goes berserk from Eve's influence. Ben, who is only 8 years old and is the son of Daniel, Aya's partner, chases after the dog and almost gets caught by monsters along the way. Once Ben catches up to Sheeva in a big empty room, he clings to the dog, wondering what is wrong with his new friend. Police chief Baker arrives and takes Ben away from Sheeva, telling Ben that Eve turned the dog into a monster. Within that moment, Sheeva mutates into a 3-headed beast, wounding Baker off-screen. Once Aya arrives, [[TearJerker Ben tells Aya that Sheeva is already dead]]. Cue BossBattle. The next day, Catherine informs you that Baker worked with Sheeva when he was a rookie (that's an old dog...) and that it must have been painful for him to shoot at her.
* In ''VideoGame/FinalFantasyAdventure'', the hero is forced to kill Amanda, a fellow gladiator, after she gets bitten by Medusa.
* In ''VideoGame/ResidentEvilCodeVeronica'', Steve is forced to kill [[spoiler:his now-zombified father]], causing him to have a brief HeroicBSOD.
* Ending C in ''VideoGame/{{NieR}}'' has the protagonist [[spoiler: killing his LoveInterest Kainé after the [[TheHeartless Shade]] part of her takes over]].
* Played with in ''GoldenSunDarkDawn'' when [[CuteMonsterGirl Sveta]] realizes that activating the [[WaveMotionGun Apollo Lens]] required to end the Grave Eclipse will [[spoiler: kill her brother, who was [[BalefulPolymorph transformed into a monster of darkness]]]] by the games main antagonists. Made worse by the fact that [[BecauseDestinySaysSo she is the only one who can do it.]]
** Subverted when he [[BigBrotherInstinct regains his memory and knocks her away from it]] so he can [[HeroicSacrifice fire it himself]], [[RedemptionEqualsDeath ending the Eclipse.]]
* ''VideoGame/Left4Dead'' provides the page image: In Zoey's backstory, she shot her father so that he would not become infected after being bit. It was only weeks later that she found out he was more likely than not immune.
* This situation comes up a few times during ''VideoGame/TheWalkingDead'', with the drama therein played to the hilt every time.
* Done in self-defense in ''VideoGame/{{Phantasmagoria}}''. [[spoiler:Heroine Adrienne is being padlocked into a torture chair which is rigged to kill her when a lever is pulled by her demon-possessed husband Don. At first, she tries to reach out to him with a snowman ornament and pleading that she loves him, but this will fail and he will pull the lever anyway if no action is taken. She ''has'' to pull the lever and make the rigging kill Don to save herself.]]
* In ''VideoGame/{{Starcarft}}:Brood Wars'' Zeratul is forced to slay [[spoiler: his Matriach Razhagal]] after it turns out [[spoiler: she was enslaved by Kerrigan]].
* [[TearJerker The most heart wrenching decision]] you'll have to make in ''VideoGame/TheWalkingDead'' and possibly in video games in general) [[spoiler:comes at the end where Lee is about to succumb to the infection that will turn him into a zombie, and the player must decide whether to have Clementine shoot him or simply run and leave him. It seems like a pretty clear cut choice and it would for most, but given the hell that poor Clementine has been through for most of the game, having to shoot the man who's faithfully protected her through it all must seem like murder. Whether she does it or not is up to the player, but either way, its hard to watch.]]
[[/folder]]

[[folder:Web Original]]
* ''Vamp You'' has included a few instances of people trying this trope, but it generally never works out. Usually, there just ends up being two vampires.
[[/folder]]

[[folder:Web Comics]]
* In the ''PennyArcade'' [[http://www.penny-arcade.com/comic/2004/7/21/ three]] [[http://www.penny-arcade.com/comic/2004/07/28/ part]] [[http://www.penny-arcade.com/comic/2004/07/30/ story]], the Cardboard-Tube Samurai is forced to kill an old ally who has been possessed by a cursed sword.
* ''Webcomic/TheAdventuresOfDrMcNinja'' has an arc where Mitzi is ''rather'' quick to do this to Dark Smoke Puncher when he gets possessed by a ghost. She's got good reason, though; for all she knows, the thing has outright killed her son and is using the body as a puppet, and she's being practical. Gordito, however, manages to drive the ghost out before that's necessary.
* The ''VideoGame/Left4Dead'' comic shows Zoey's back story that occurred a week after the zombie outbreak. A zombie breaks into the apartment and bites Zoey's mother. She gets infected and attacks Zoey's father, which forces him to shoot her dead. Fearing that he is now infected, Zoey's father asks her to shoot him, which she tearfully does so. This scene is more gut wrenching when Zoey discovers that the carrier gene (what prevents people from becoming a zombie) is passed down on the father's side on the family, [[SenselessSacrifice thus there was a chance Zoey's father could have been just fine]].
[[/folder]]

[[folder:Real Life]]
* This can happen to anyone who grows up good friends with someone from a foreign country whose government goes to war with the government of the first person's country, or who has a friend who switches sides in any war or gang dispute. (Compare and contrast with StarCrossedLovers.)
** Very often, this is how the UsefulNotes/AmericanCivilWar is regarded.
* There have been cases where people who have been driven violently insane by rabies have attacked their own loved ones and had to be killed in self-defence.
** The same could apply to any sickness or psychosis that could make someone dangerous to their own loved ones.
[[/folder]]

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