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->''"There's always that one guy on the team too, he was a last minute replacement. He's not one of the original gang.\\
But one of the other guys vouches for him. 'No, no, dude... trust me, this guy's cool. He's solid and he's cool.' But he's not cool, is he? He doesn't really say anything ever, right?\\
He just stands there, looks cool... and then at one point he might be like... 'Let's kill these bitches.'"''
-->-- '''Creator/DaneCook'''

Getting along with a group of people in everyday life is hard, working together with a group of people under stressful situations can be downright murder.

But that's where all the tension and drama is, right?

A group of survivors is rarely written as a cohesive group, or if they are, there's one little catch. One of them is emotionally fragile, and given the right situation, will have a psychotic breakdown, usually due to crossing the DespairEventHorizon. So now the group of survivors has to be wary of not only external threats, but an internal one as well, from someone who could possibly slaughter everyone in the group without knowing it. This character isn't (normally) evil, but they are a convenient obstacle for the protagonists. Usually, there's a tough decision on whether or not to kill the person in question. When it's someone whom you least expect to flip out, the trope becomes that much more chilling.

These characters come in two types:

* Those who had '''One Bad Day''': These are seemingly normal people who for whatever reason becomes mentally and or emotionally unhinged and becomes a danger to the group. Before hand they were seemingly normal and sane.
* Those who are '''One Bad Apple''': These characters are probably loose cannons who goes over the moral line with their brutality and or hasty behavior. Usually overlaps with TokenEvilTeammate.

Could be a result of a [[GoMadFromTheRevelation maddening revelation]].

Contrast with TokenEvilTeammate, Could also overlap with AHouseDivided, and ZombieInfectee.

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!!Examples:

[[foldercontrol]]

[[folder:Anime & Manga]]
* ''Anime/MobileSuitGundamZZ'' has Elpeo Puru. Normally she's a [[GenkiGirl hyperactive young girl]] with a sisterly attachment to Judau, but sometimes her obsession with him can go a bit... far. [[spoiler:The worst case was when she tried to kill Leina, seeing her as a rival for Judau's brotherly affections. Judau manages to stop this and calm her down, and Puru is heartbroken over Leina's supposed death a short time later.]]
[[/folder]]

[[folder:Comic Books]]
* The opinion of several members of the resistance during Marvel's ComicBook/CivilWar of ComicBook/ThePunisher. ComicBook/CaptainAmerica even calls him insane after he throws him out for gunning down a pair of C-list villains who wanted to join.
* ''ComicBook/TheWalkingDead'' has quite a few of these.
** Specifically [[spoiler: Ben brutally slaughters Billy, and before then he was seen killing a cat,]] and of course the group goes through the whole should we or shouldn't we kill him debate. Because MurderIsTheBestSolution. [[spoiler: Carl of course makes the decision for them while everyone was preoccupied.]]
** Michonne, and even Rick teeters on the brink of this sometimes, and possibly [[spoiler: Morgan and Carl.]]
%%** And of course, Carol.
** Shane as well. Doesn't really helps that he was ''literally'' TheSociopath.
* Whirl from ''ComicBook/TheTransformersMoreThanMeetsTheEye''. He's an AxCrazy {{Jerkass}} who is initially kept around because his skill and willingness in combat makes him useful to the crew. However this causes some problems as Rodimus learns the hard way that Whirl isn't easy to control. For instance, waiting until the crew's backs are turned than chasing down and torturing info out of a mook against orders.
[[/folder]]

[[folder:Films -- Live-Action]]
* Ash from ''Film/{{Alien}}''. Interestingly enough, at least ''part'' of the reason he's the PsychoPartyMember is because [[spoiler:[[MegaCorp Weyland-Yutani]] (a.k.a. "The Company") ordered him to bring the alien back to Earth, [[CorruptCorporateExecutive and everybody else aboard the Nostromo was to be considered expendable]].]]
* Quentin from ''Film/{{Cube}}''. The biggest threat to everybody in their escape plan is him, who will kill them all ''just because he doesn't likes them'', and when they try to leave him behind... oh, boy.
* Archer Maggot from ''Film/TheDirtyDozen'' is a Type II. He's clearly off-balance from the start, spouting off racist comments like nobody's business, but he really becomes a problem at the climax where he nearly foils the mission by stabbing a woman to death at the party... for no reason.
* Andre from Creator/ZackSnyder's ''Film/DawnOfTheDead2004''. The man wants to bring his kid into the world, ok, that is nice... and then we find out that his wife (and maybe the baby) are [[ZombieInfectee Zombie Infectees]], and he doesn't gives a damn about this...
%%* Pvt. Miguel Salazar from ''Film/DayOfTheDead1985''.
%%** Logan as well.
%%* Ridley from ''Film/DiaryOfTheDead''.
* Dr. William Weir from ''Film/EventHorizon''. For the most part he was still sane up until the film's climax. In fact he was genuinely remorseful when he came across [[spoiler: Peters body]].
* Ian from ''Film/FinalDestination3'' snaps due to the loss of his girlfriend. He then tries to kill his friends to steal their time and get off of Death's list.
* Waingro in ''Film/{{Heat}}'', who the page quote is an indirect reference to. He becomes such a kill-happy madman that [=McCauley=] not only doesn't laments him dying on the climactic heist, but he wastes precious getaway time to hunt down the one who recommended Waingro to be part of the team.
* [[spoiler:Corporal Gallo]] from ''Film/{{Pandorum}}''. He's the reason the whole mess with the mutants is going on. [[spoiler:While it's not completely hard to understand that finding out that the people aboard the ship to Tanis ''are the last humans alive'' would trigger an episode of Pandorum, him killing the rest of the crew and [[AGodAmI literally playing God]] with the colonists was too far.]]
* ''Film/PanicRoom'': Raoul. Things would really have been a lot better if Junior hadn't gotten him involved.
* Mr. Blonde from ''Film/ReservoirDogs''. His response to someone hitting the alarm during the heist that makes up the movie was to go on a kill-crazy rampage, and so it's little wonder that a good 90% of anyone else's dialogue about the man is how psychotic he is. And that's before we get to the ColdBloodedTorture scene...
%%* Xavier from ''Film/SawII''.
* Pinbacker from Danny Boyle's ''Film/{{Sunshine}}'', [[spoiler:who lost his marbles and killed his crew members]].
* Blair from John Carpenter's ''Film/TheThing1982''. Although there's a good chance he was right to act so crazy. At first, anyway.
* Harlan Ogilvy from Spielberg's ''Film/TheWarOfTheWorlds''. The man wants to dig a tunnel from his middle-of-nowhere house towards the nearest city and start a campaign of guerrilla warfare against the aliens (which looks like a HopelessWar ''with a fully staffed and equipped army'', let alone Oglivy's 12-gauge) and when the aliens enter the house, he readies the gun and prepares to fight back, even when it would probably end up killing everybody ([[PapaWolf including Ray's daughter -and there is no way in hell that he will let him risk that]]).
* Goke from ''Film/TheZombieDiaries'', but he may have always been nuts.
* The ComicBook/IncredibleHulk in ''Film/{{The Avengers|2012}}'', at least during the first two acts. He's a smash-happy beast [[spoiler:and an important part of Loki's plan is to manipulate the emotions of the Avengers and set off Banner's UnstoppableRage in a place where nobody can run away from him (the Helicarrier).]]
* Snyder from the film ''Deep Star Six''. When a monster attacks various members of the crew, he's given the order to get the site ready for evacuation -so Snyder looks into the evac protocols under "site being threatened by enemy force" and follows them to the letter... without caring that these protocols include ''setting off the nukes that are within spitting distance of Deep Star Six'' and they hadn't left the place yet. When the massive underwater nuclear explosion inevitably damages the station, he insists on trying to leave the place even when everybody else tells him that leaving without decompressing would mean suicide... [[DirtyCoward and still he does, the very second that it looks like trying to face the monster will not work]]. [[ExplosiveDecompression No prizes]] [[ChunkySalsaRule for what happens to him next]]. Foreshadowed By the fact he was over worked and suffering from some form of cabin fever. But for whatever reason his superior wouldn't give him a break.
* A DefiedTrope on ''Film/TwilightsLastGleaming'': when it becomes apparent that Hoxey is one of these (killing a couple of guards while they are being MuggedForDisguise because, in his words, "I don't like officers" -bear in mind that he was asked not to do that), General Dell waits until they are within the silo's primary entrance to kill Hoxey. [[DidNotThinkThisThrough The problem for Dell then becomes]] that Hoxey was the only guy of the team that knew how to do SafeCracking, and so they need to improvise a way to get the combination from the silo's personnel/hostages so they can launch the missiles...
* ''Film/TheWildBunch'': The opening heist has one member of the titular Bunch that stays behind during the getaway to keep a look over the hostages... and is unhinged enough to lick the neck of the sole woman of the group and force them to march around the office they are being held in before unloading his shotgun unto them. The rest of the Bunch not only do not do LeaveNoManBehind, but very much do not give a damn about him (and the fact that Thornton's posse killed him).
[[/folder]]

[[folder:Literature]]
%%* Aarfy from ''Literature/CatchTwentyTwo''.
* Craig Toomy in Creator/StephenKing's ''Literature/TheLangoliers.'' He's already bubbling on the verge of a breakdown before the story heats up, but he's pushed over the edge by the [[GoMadFromTheRevelation maddening revelation]] that [[spoiler:the stale-time-eating Langoliers he heard stories about in his youth are real.]]
* Tedrin in ''Literature/EdenGreen'' is deviously insane due to the alien needle symbiote that has taken over his brain and body. The main character never fully trusts him, while her best friend (his girlfriend) believes he can be redeemed.
[[/folder]]

[[folder:Live-Action TV]]
* [[MysteriousWaif River]] is the Psycho Party Member in ''Series/{{Firefly}}''. After her [[MindRape treatment]] at the hands of the Alliance government, she is only sometimes sane and is apt to flip out at the slightest (if any) provocation. Most of the crew realize that her behavior is not her fault, and are protective toward her, but they do remain wary. The episode ''Objects in Space'' revolves a lot around the other characters beginning to seriously consider the threat River poses to them, and their [[ParanoiaFuel concern is increased]] by ''Film/{{Serenity}}'' when they find out that River was intended to be an [[TykeBomb elite assassin]].
* Despite it being about a group of survivors under ''very'' stressful conditions, ''Series/{{Lost}}'' averted this. Though Michael came ''so close'' in early season 2.
* Happens twice in ''Series/ThePacific''. On Peleliu an unnamed Marine begins to freak out one night, and his comrades have to resort to bashing his head in with an entrenching tool to silence him. Later on Okinawa, Pvt. Peck snaps as a result of the rain, Japanese shelling, and hazing he's been receiving from Snafu, Sledge and Leyden, and begins screaming at the Japanese position across from them, even firing a few shots in their direction. While the others attempt to calm him [[spoiler: another replacement having better luck fitting in with the others, Pvt. Hamm, is shot and killed by the Japanese responding to Peck's breakdown]]. TruthInTelevision.
* Ronald Speirs in ''Series/BandOfBrothers'' has this vibe going, and actually [[CrazyAwesome makes it work for him]]. His comrades speculate whether he's ''really'' as psychotic as the stories make him, or [[AlternateCharacterInterpretation if it's all just an act]]. As with the above, this is also TruthInTelevision, and to this day no one is really sure whether the stories told about Speirs during the War were true or if it was an image he allowed his subordinates to cultivate. He was certainly well-respected and [[AFatherToHisMen popular]] among the men of [[BadAssCrew Easy Company]].
[[/folder]]

[[folder:Theater]]
* In ''Theatre/BatBoyTheMusical'', [[spoiler: Dr. Parker's already somewhat loose grip on reality is shattered when his wife takes Edgar's side over his, and he begins killing people and framing Edgar for the crimes.]]
[[/folder]]

[[folder:Video Games]]
* In ''VideoGame/StarWarsRepublicCommando'' Scorch worries that Sev may be one of these in one of the loading screens:
-->'''Scorch:''' Boss, Keep an eye on Sev. He's been talking in his sleep lately, and I gotta tell you... it's scary stuff. Very scary stuff.
* [[ByronicHero Anders]] of ''VideoGame/DragonAgeII'', due to being a [[DemonicPossession demonically possessed]] WellIntentionedExtremist.
* In a reference to Ash from ''Film/{{Alien}}'', above, [[NamesToRunAwayFromReallyFast Darth]] in Cube's chapter of ''VideoGame/LiveALive''. [[spoiler:He turns out to be not only a good guy, but one of only two human survivors.]]
* While a number of the hunters in ''VideoGame/{{Evolve}}'' aren't exactly stable, Kala takes the cake. On her own, she's a perfectly reasonable and intelligent woman. Unfortunately for everyone else she ''isn't'' on her own because she's connected to the monster HiveMind and is slowly mutating, so that not only is there the possibility that's she'll snap and attack them but a certainty that when that happens she'll transform into an alien killing machine.
* Trevor Phillips is full blown nut case and drug dealer in ''VideoGame/GrandTheftAutoV''. While Franklin and Michael are more collected and rational and kill only people they are assigned to kill, Trevor will kill ''anyone'' for ''any reason'' that makes sense only to him. He can be a screaming lunatic that strongarms his friends into his schemes one minute and be calm and rational the next minute in the blink of an eye. Trevor prefers to kill witnesses during a heist so that no one can trace him while Michael prefers to keep needless kills to a minimum. Regardless of Trevor's sketchy persona, he's an excellent gunman and a damn good pilot, which are quite valuable assets for the many jobs that the protagonists take. After Trevor finds out about Michael's secret where [[spoiler: he sold Trevor and Brad out after a failed bank heist years ago in order to save his own skin due to having a family]], Trevor keeps loudly debating on whether or not he should really kill Michael over it and that creates a big rift between the two of them with Franklin being caught in the middle of the spat and being agitated from it. [[spoiler: One of the endings has Franklin killing Trevor in order to save his own skin (just like what Michael did to Trevor) while trying to justify it by pointing out that Trevor's behavior was going to get the whole group killed.]]
[[/folder]]

[[folder:Visual Novels]]
* Although it's hard to say for sure under all the UnreliableNarrator issues and MindScrew, Maria in ''VisualNovel/UminekoWhenTheyCry'' is definitely a little unbalanced. It's particularly clear in the first and fourth arcs; in the former, she doesn't react at all to the gory murders, and in the latter, she [[spoiler:kills (or at least appears to kill) her mother over and over and over in the meta-world.]]
* The [9]th Man of ''VisualNovel/NineHoursNinePersonsNineDoors'' splits in as soon as the Nonary Game begins. He takes Clover hostage with a knife, and demands for their co-operation in him entering a door. By doing this, though, he breaks the rules of the game and dies horribly. Not only him, but [[spoiler: Ace, panicking after TheReveal, and Clover, who goes AxCrazy after her SanitySlippage of losing her brother.]]
[[/folder]]

[[folder:Web Comics]]
* Although he literally fits the trope name, Belkar of ''Webcomic/TheOrderOfTheStick'' is usually the TokenEvilTeammate rather than this trope. [[DiscussedTrope As is frequently noted in-story]], this is because of [[TheHero Roy's]] ability to keep him in line. When [[spoiler:Roy dies and [[TheLancer Haley]] becomes the party leader]], Belkar moves into this trope. The first demonstration is when he randomly kills an innocent gnome merchant whom they meet on the road. When he then kills [[spoiler:[[TheOmniscient the Oracle]], whose help the party needed]], it's the last straw.
%%** [[InsufferableGenius Vaarsuvius]] fits as well, some of the time.
* Porkfry is/was this apparently for the ''Webcomic/PennyArcade'' crew, according to [[http://penny-arcade.com/comic/2003/09/05 this comic]] where he suggests the best response to Tycho standing them up for Guys' Night Out is to kill him (Complete with brandishing a pocket knife)
-->'''Porkfry:''' ''[disappointed]'' We never do what I want to do.\\
'''Gabe:''' Listen, if you wanted to go to Dairy Queen or something, fine, let's go. But no. You always want to fucking kill people. You never just want a Blizzard.
[[/folder]]

[[folder:Web Original]]
%%* WebVideo/EverymanHYBRID brings you [[KnifeNut Evan]].
* The list of ''Blog/ThingsMrWelchIsNoLongerAllowedToDoInAnRPG'' contains some rather terrifying insights on the mind of the PsychoPartyMember.
-->1683. [[AMillionIsAStatistic Killing the orc horde by drowning them all at once is heroic]]. Killing them by drowning them one at a time is an alignment check.
[[/folder]]
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