->''"Fact: The key to any successful cooperative test is trust. And as our data clearly shows, [[HumansAreBastards humans cannot be trusted.]]"''
--> -- '''Cave Johnson''', ''VideoGame/{{Portal 2}}''

A Team Killer is a person who murders other people on their own team.

If you are a member of a group of people dedicated to doing dangerous work, you're probably depending on the other members of your group to back you up. You might not [[TeethClenchedTeamwork like them]], but you'll trust them at least enough not to shoot you in the back. That way, when they shoot you in the back anyhow, the betrayal just makes it all the more shocking.

Villains might do this to each other for any number of reasons, because villain team-ups are generally unstable things at best. Villainous team-killing is most often done to [[KickTheDog demonstrate]] [[ObviouslyEvil how evil someone is]]. Heroes normally don't team-kill, or at least believe that they shouldn't, because they depend on the PowerOfFriendship to bring them through things that would destroy less unified groups.

PlayedForDrama, a Team Killer is usually a reprehensible person, who everyone despises as a traitor. Team Killing is often a MoralEventHorizon. A pre-emptive strike to stop a party member from doing something despicable is even more tragic. Occasionally Team-killing is depicted as NecessarilyEvil, but even then it's nearly always morally ambiguous at best.

PlayedForLaughs, a Team Killer is a {{comedic sociopath|y}} taken to its logical extreme, and chances are pretty good that the Team-Killed deserved it.

The term originates in online multiplayer FirstPersonShooter games, where certain adolescents take joy in deliberately killing their team-mates. "What the hell, man? Don't team-kill!" is used as an admonishment to this very day. Team killing tends to earn the ire of server admins as well, which can result in the team killer being kicked and/or banned from the server.

A Team-Killer who kills his own employees might be a BadBoss; see YouHaveFailedMe and YouHaveOutlivedYourUsefulness for elaboration. In large amounts, it's WeHaveReserves.

See BetrayalTropes for a list of many of the people, reasons, and methods involved in Team Killing. For when Team Killing is done in a strictly meta-videogaming sense, see {{Griefer}} or PlayerKilling.

No real-life examples, please. In real life, the matter of who's on what "team" tends to be a lot less distinct than it is in fiction.

[[AC:{{Anime}} and {{Manga}}]]
* ''{{Berserk}}'': Both Guts and Griffith.
** Guts, in a sympathetic example, did this when he was just a kid to one of his adoptive father's men during a battle as revenge for raping him.
** Griffith's case involved [[spoiler:sacrificing his entire goddamned army to become the fifth member of the Godhand]].
* ''RaveMaster'': [[AxCrazy Lance]] kills [[EliteMook Bis]], one of his underlings, apparently because he was taking too long to kill an enemy that Lance wanted a turn with.
* ''Anime/DragonBallZ'': Vegeta kills [[spoiler:Nappa]], his fellow Saiyan, when the latter is defeated by Goku.
** And the series started off with Piccolo pulling this on Goku, though partly because he ''had'' to to finish off Raditz. ([[ItsPersonal He still wanted to kill Goku himself.]])
* In ''{{Robotech}}'', Khyron [[NamesToRunAwayFromReallyFast the Backstabber]] earns his title not from ChronicBackstabbingDisorder, but because he tends to [[TheNeidermeyer shoot his own men]] when they run or when they're in his way.
* ''Anime/PuellaMagiMadokaMagica'': [[spoiler:In one of the past timelines]] episode 10, Mami killed Kyoko in fear that they would eventually [[spoiler:turn into witches like Sayaka]]. Madoka killed her in return before she could cause any further deaths.
* ''BrokenBlade'': Girghe had no issues backstabbing his own team mates for no reason whatsoever.
* ''Manga/{{Naruto}}'':
** Sasuke is a team killer.
*** Sakura genuinely cared for his well-being and always did her best to support him. And how does he repay her? [[MoralEventHorizon By trying to electrocute her, strangle her and stab her into the face within the same five minutes.]]
*** Karin was Sasuke's teammate in Taka - until he decided that she had [[YouHaveOutlivedYourUsefulness outlived her usefulness]]. It's made even worse by the fact that she [[UngratefulBastard saved his life multiple times and helped him figure the enemy's attacks out]] mere minutes beforehand.
** [[BitchInSheepsClothing Mizuki]], the first antagonist of the series, was revealed to be this in a filler arc when it was shown that he killed his teammates during a mission for being weak.
** Kakuzu often loses his temper and kills anyone nearby, which as a member of Akatsuki usually resulted in him killed his partner. Eventually he was paired with Hidan because, even if [[TeethClenchedTeamwork they hated one another]] Kakuzu wouldn't be able to kill him.
** Kisame did this to a group of code-breakers from the Mist village to prevent them from falling into the enemy's hands. When he gets teamed up with Itachi, he makes note of how they both massacred their comrades.
** Pre-HeelFaceTurn Gaara makes death threats against his teammates (who happen to be his older brother and sister) when they object to taking unnecessary risks in the Forest of Death.
* Comes up a ''lot'' in ''Manga/{{Bleach}}'' among the villains. Grimmjow Jaegerjaquez lost his rank when he lost his arm (that held his rank tattoo) and was replaced by [[spoiler: Luppi Antenor]]. Grimmjow came back and the two fought over the rank, with only one surviving. Zommari Rureaux and Nnoitra Gilga did this to their fellow espada. Rudbornn Chelute was an arrancar whose ''job'' was to slay defeated arrancar. And Giselle Gewelle did this to [[spoiler: her former boss]].
** Pepe Waccabrada [[InvokedTrope tried]] to do this to Liltotto Lamperd with a [[BrainwashedAndCrazy brainwashed]] [[SuperStrength Meninas McAllon]] [[ForTheEvulz just because]]. [[spoiler: Unfortunately for him, [[ImAHumanitarian Liltotto]] survived and [[KickTheSonOfABitch payed him back in kind.]]]]

* ''ComicBook/{{Invincible}}'': This is how we find out that Omniman is not quite the hero that everyone thinks he is.

* ''Film/{{Aliens}}'': Burke attempts to have Ripley and Newt impregnated by a facehugger in order to smuggle alien embryos back with him, which would render them ticking time bombs. Thankfully, he fails.
* In ''Film/GuardiansOfTheGalaxy'', Ronan kills The Other for lecturing him on showing respect in Thanos's presence.
* In ''HornetsNest'', IgnoredExpert Captain von Hecht shoots SS officer Taussig after a heated spat that ends with Taussig refusing to acknowledge the danger the dam is in. He then lies to Taussig's MookLieutenant and says "The shock of the incident was [[DrivenToSuicide too much]] for the Major."
* In ''The Bridge At Remagen'', Major Kreuger shoots two of his men who try to run away. [[spoiler:He's later executed himself by the SS.]]
* All of the Joker's team at the beginning of ''Film/TheDarkKnight''.
* In ''[[TheDirtyDozen The Dirty Dozen: Next Mission]]'', there's a very bizarre scene on the train wherein General Dietrich shoots his aide Schmidt. [[spoiler:Apparently Schmidt was beginning to suspect Dietrich intended to assassinate Hitler, and Dietrich shoots him to keep him quiet, but this is never explicitly stated.]]
* ThoseWackyNazis sure do love doing this. SS Major Kaempffer blows Captain Woermann away in the film version of ''Film/TheKeep''. [[spoiler:They'd just had a heated argument, and then heard gunshots outside. Although never explicitly made clear, Kaempffer had apparently been looking for an excuse to kill Woermann for constantly criticizing him. As a bonus, he also seemingly did it to steal Weormann's cross in a doomed effort to protect himself from Molasar.]]

* Literature/TheBible: Judas is so infamous that his name is now a synonym for "backstabbing traitor." He's the inspiration for the FindingJudas trope.
* ''GauntsGhosts'': Lijah Cuu [[spoiler:kills troopers Bragg and Meryl and tries to murder Saint Sabbatine herself before being brought down...but not before killing fan favourite ColonelBadass Colm Corbec in the crossfire.]]

* One of the ongoing subplots in ''{{Firefly}}'' is Jayne's willingness to sell out TheTeam, specifically River and Simon. The subplot is resolved when [[spoiler:Mal finds out about Jayne's attempt to turn in the Tams to the Feds on Ariel and almost [[ThrownOutTheAirlock throws Jayne out the airlock as the ship is taking off for it.]]]]
* ''Series/{{Battlestar Galactica|Reimagined}}'': Tragically and repeatedly due to Cylon infiltrators.
* On ''{{NCIS}}'', poor [[spoiler:Special Agent Lee]], a rare semi-sympathetic version, was forced to kill [[spoiler:Langer]] after [[IHaveYourWife a loved one was kidnapped.]]
* The previous leader of Series/{{Torchwood}} Three [[MurderSuicide killed his entire team and then himself]], leaving Jack Harkness to rebuild the team.
* In ''Series/PowerRangersNinjaStorm'', Vexacus kills both Zurgane and Motodrone as part of his plan to seize control of Lothor's ship.

* {{Munchkin}}s do this to steal exp and items.
* Most factions in TabletopGame/{{Warhammer 40000}} do at least a little bit of Team Killing. The Orks and Chaos in particular have built societies on it. Specific characters known for being Team Killers include:
** [[NamesToRunAwayFromReallyFast Kharn the Betrayer]] earned his name this way.
** TheCommissar is supposed to kill the men in his company, and for any reason he wants.
** {{Horus|Heresy}} is worse than standard team kills because this team was his ''family''.
* [[Blog/ThingsMrWelchIsNoLongerAllowedToDoInAnRPG Mr. Welch]] has occasionally forged [[WeaponOfXSlaying Weapons Of X Slaying]] to take out particularly annoying party members.

* Pick any team-based multiplayer game where FriendlyFireproof is not enabled. {{Griefer}}s are often [[PlayerKilling team-killers.]]
** In ''WorldOfWarcraft'', it's possible to use certain encounter mechanics to kill your fellow party members (for example, standing next to someone with a debuff that harms everyone nearby). Most people do this by accident, but the potential for deliberately killing someone exists.
** [[https://m.youtube.com/watch?v=JoDLZhZZOIw One video]] showed a ''TeamFortressTwo'' server that briefly deactivated FriendlyFireproof. The second the players realized what had happened, they began shooting their teammates.
* Many traditional RPG's such as ''Franchise/FinalFantasy'' allows you to KO'd your own team members, and many are willing to do so for reasons such as protagonist-based LevelScaling, CantDropTheHero, and TheScrappy that pointlessly reduces EXP rewards.
** In certain Final Fantasy games, certain moves or abilities will only work if other people in the party are knocked out. This can lead to players intentionally attacking their own team just to get the effects they were seeking.
* It's a common element in the ''Franchise/{{Disgaea}}'' games that killing too many of your own units, intentionally or not, often locks you out of the better endings.
* ''[[VideoGame/DissidiaFinalFantasy Dissidia 012]]'' has [[spoiler:[[VideoGame/FinalFantasyIV Kain]]]] playing the WellIntentionedExtremist variant of this. He also convinces [[spoiler:[[VideoGame/FinalFantasyI Warrior of Light]]]] to join in on the plan, and they're both [[WhatTheHellHero chewed out quite extensively]] by [[VideoGame/FinalFantasyXIII Lightning]] when the betrayal is revealed. [[spoiler:Kain]] has ''really'' good reasons for doing so, however; [[spoiler:[[VideoGame/FinalFantasyIV Golbez]] tipped him off that if the [[HumanoidAbomination manikins]] defeat any of the warriors during the war, they'll be KilledOffForReal, as the gods will not be able to revive them. This is why the likes of Lightning, Kain, [[VideoGame/FinalFantasyXII Vaan]], [[VideoGame/FinalFantasyVIII Laguna]], [[VideoGame/FinalFantasyVII Tifa]], and [[VideoGame/FinalFantasyX Yuna]] are not present in the first game; [[HeroicSacrifice they died ensuring that the Rift would be closed, preventing a huge influx of manikins into the cycle of war]].]]
* In ''VideoGame/BaldursGate'', if you recruit Minsc, Dynaheir, and [[ArchEnemy Edwin]], the former two and the latter will eventually go at it.
* There are several points in ''VideoGame/DragonAgeOrigins'' where you can kill party members (or party members can kill you) as a consequence of "ethical disagreements".
* The same can happen in another Bioware production, ''Franchise/MassEffect'', the most glaring potential examples being [[spoiler:Samara]] in the second game: you can kill her off just like that and employ [[spoiler: her murderous daughter]] to fulfill the same role on the team. There is no substantial loss nor gain from doing so.
* In ''VideoGame/JadeEmpire'', the player character can do this to about half of his/her party.
* In ''VideoGame/{{Metroid}}: [[VideoGame/MetroidOtherM Other M]]'', there is a mysterious character known as the Deleter. It is clad in armor identical to that of the Galactic Federation soldiers Samus works with over the course of the story and was sent with the 07th Platoon to kill all witnesses to prevent them from discovering the links between the Bottle Ship and the Federation. While the subplot is dropped before being fully solved, it's heavily implied that the person in question is [[spoiler:James Pierce, the communications expert of the squad.]]
* In ''VideoGame/NieR'', [[spoiler: the final ending provide you the choice: either kill Kainé, putting her out of her misery, or give up your own life to get rid of the Shade possessing her.]]
* In ''UltimaV'', Saduj is a joinable NPC who will attack your other party members the minute you enter combat with him in the party. This pretty much serves you right, not only is he an open supporter of King Blackthorn whom your party is actively working to overthrow, but [[SdrawkcabName his name is clearly "Judas" spelled backwards]].
* The ''Videogame/{{Portal 2}}'' quote atop the page comes from a preview trailer for the game, where one stick figure pushes another into a fire for no reason. The solution: robots! After rigorous trust exercises, robots are able to work together for a impressive ''six seconds longer''. In-game, there are several co-op achievements that involve deliberately murdering your partner.
* In ''Septerracore'' putting certain combinations of characters in your party is ill-advised, as they ''will'' occasionally swipe at each other for real damage.

* [[Webcomic/EightBitTheater Black Mage.]] Sometimes Red Mage or Thief, but mostly Black Mage.
* Referenced in ''Webcomic/PennyArcade'', when [[ItMakesSenseInContext the Twelve Apostles are playing]] ''Franchise/{{Halo}}''.
-->'''Peter:''' Christ, Judas is team killing again.
-->'''Christ:''' Judas, don't be a dick.
* Yeon from ''Webcomic/TowerOfGod'' is a fire mage that regularly fries her own team members by accident, among them Ja Wangnan, the main character whose life is in danger because he keeps failing the tests. When they meet yet again, HilarityEnsues.
* There's no ''fatalities'' in ''Webcomic/TheOrderOfTheStick'', but [[TriggerHappy Vaarsuvius]] and [[HeroicComedicSociopath Belkar]] have come close on occasion.
-->'''Roy''': Dammit, Vaarsuvius, I've told you before, we do NOT blow up members of our party!
-->'''Roy''': ''(to Belkar)'' If you kill Elan for [=XP=], we'll kill ''you'' for [=XP=].

[[AC: WebOriginal]]
* In ''RedVsBlue'', Caboose accidentally killing Church was something of a running gag. Whenever they want Caboose to kill somebody, they just tell him to "help" the enemy. He has the largest body count of the Blue and Red Teams and ''all of them were team kills''. He nearly depopulated a Blue Army base off-screen. Command ''even has a keyboard shortcut'' for recording Caboose's team kills: [[StealthPun Ctrl+F+U.]]
** Caboose accidentally killing Church (the first time)? ''[[TimeyWimeyBall Church's fault]]''.
-->'''Church:''' Oh, my god. ''I'm'' the team-killing fucktard!
*** Granted, it was Gamma who tortured Alpha with simulations in the past, so it could have been Gary/Gamma simply screwing with him.
** Sarge will take every opportunity to kill Grif.
* Vegeta (unsurprisingly - see above) in ''LetsPlay/TwoSaiyansPlay''. He's especially trigger-happy when he and Nappa play ''VideoGame/{{Magicka}}'', and indulges in the EvilLaugh while doing so.
* ''Series/{{Noob}}'' has shown this a couple of times:
** In Season 4, [[spoiler:Gaea ends up factionless in presence of her guild and a couple of fanboys of hers from the enemy faction. Her FaceHeelTurn happens via her getting her fanboys to form a PickUpGroup with her and killing her guildmates to get reputation with her new faction]].
** In Season 5, Roxana doesn't let a bunch of members of her faction stand in the way of a chance to deal decent damage to the game's BigBad.

[[AC: WesternAnimation]]
* ''WesternAnimation/TotalDrama''
** Trent was one for the Killer Grips in ''Total Drama Action'', thanks to Owen giving him some bad advice.
** Courtney becomes one in ''World Tour'' in order to get a chance to vote off Gwen.
** Scott is a characteristic example in ''Revenge of the Island''. He seems more interested in eliminating his own teammates than the other team. He's sneaky enough that he can pull this off without getting booted. As of ''All Stars'' he drops the act and starts playing fair.