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.'"
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 Despair Event Horizon. 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. Beforehand, they were seemingly normal and sane.
- Those who are One Bad Apple: These characters are probably loose cannons who go over the moral line with their brutality and/or hasty behavior. Usually overlaps with Token Evil Teammate.
Could be a result of a maddening revelation.
- Mobile Suit Gundam ZZ has Elpeo Puru. Normally she's a hyperactive young girl with a sisterly attachment to Judau, but sometimes her obsession, with him can go a bit... far. 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.
- The opinion of several members of the resistance during Marvel's Civil War of The Punisher. Captain America even calls him insane after he throws him out for gunning down a pair of C-list villains who wanted to join.
- The Walking Dead has quite a few of these.
- Specifically, 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 Murder Is the Best Solution. 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 Morgan and Carl.
- Shane as well. Doesn't really helps that he was literally The Sociopath.
- Whirl from The Transformers: More Than Meets the Eye. He's an Ax-Crazy 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.
- Ash from Alien. Interestingly enough, at least part of the reason he's the Psycho Party Member is because Weyland-Yutani (a.k.a. "The Company") ordered him to bring the alien back to Earth, and everybody else aboard the Nostromo was to be considered expendable.
- The Incredible Hulk in The Avengers, at least during the first two acts. He's a smash-happy beast and an important part of Loki's plan is to manipulate the emotions of the Avengers and set off Banner's Unstoppable Rage in a place where nobody can run away from him (the Helicarrier).
- The 2014 movie Black Sea has Fraser, who is mentioned from the get-go to be a "psychopath" and shortly after is said to have been coming in and out of jail for some time now; but he still is recruited for the recovery mission/heist at the bottom of the titular Russian ocean because his capacity as a salvage diver makes him necessary ("he's half-fish"). The two times the crew's sub sinks and nearly kills everybody is because he loses his head and kills another member.
- Quentin from 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.
- Snyder from the film DeepStar 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" (whereas "threatened by forces of nature" was also a plausible choice) 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... and still he does, the very second that it looks like trying to face the monster will not work. No prizes 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.
- Archer Maggot from The Dirty Dozen 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 Zack Snyder's Dawn of the Dead (2004). 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 Zombie Infectees, and he doesn't gives a damn about this...
- Dr. William Weir from Event Horizon. For the most part he was still sane up until the film's climax. In fact he was genuinely remorseful when he came across Peters body.
- Ian from Final Destination 3 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 Heat starts the movie as member of a professional crew of thieves, but becomes such a kill-happy madman that Consummate Professional Neil McCauley risks attempting to execute him in public after their first heist turns into a bloodbath due to Waingro's sadism. After Waingro escapes he allies with Neil's enemies in the underworld and does everything he can to bring Neil down, but then goes above and beyond when he rapes and kills the wife of a member of Neil's crew and beats the crew member to near death when the guy attempts to avenge her, all for no reason other than because Waingro is an uncontrollable nutcase. By this point Neil is so furious with the psycho that despite being the subject of a police manhunt Neil takes time out of his getaway to hunt down Waingro and kill him properly.
- King of Thieves: Terry Perkins is an old school London criminal with a hair trigger temper, who—at the drop of a hat—will decide someone is slighting him or trying to cheat him. His acts include smashing a chair in the floor and using it to threaten Basil out of his share of the loot, and dousing Carl in petrol and threatening to set him alight if he talks or makes any claim on the proceeds.
- Corporal Gallo from Pandorum. He's the reason the whole mess with the mutants is going on. 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 literally playing God with the colonists was too far.
- Panic Room: Raoul. Things would really have been a lot better if Junior hadn't gotten him involved.
- Mr. Blonde from Reservoir Dogs. 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 Cold-Blooded Torture scene...
- Xavier from Saw II. He tosses everybody else to the wolves and when he discovers the clue to get out of the death trap-laden house has been tattooed in pieces on the back of everybody's heads, he starts chasing the rest to kill them and get their piece.
- Pinbacker from Danny Boyle's Sunshine, who lost his marbles and killed his crew members.'
- The Taking of Pelham One Two Three: Mr. Grey is the most trigger-happy and violence-prone member of the group that has taken over the titular subway train, to the point that the leader of the group, Ryder, has to blow him away when Grey utterly refuses to follow a step of the plan wherein they get rid of their guns to mix amongst the New York crowd as they escape, knowing that this will inevitably lead to a shootout. Grey is also mentioned in the original novel to have been a Mafia member, but he had been expelled from the Mafia because his constant acts of violence got in the way of their racketeering — because knowing that Grey was going to brutalize them whether they paid or not, the people just decided to not pay anymore.
- Blair from John Carpenter's The Thing (1982). Although there's a good chance he was right to act so crazy. At first, anyway.
- A Defied Trope on Twilight's Last Gleaming: when it becomes apparent that Hoxey is one of these (killing a couple of guards while they are being Mugged for Disguise 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. The problem for Dell then becomes that Hoxey was the only guy of the team that knew how to do Safe Cracking, and so they need to improvise a way to get the combination from the silo's personnel/hostages so they can launch the missiles...
- Harlan Ogilvy from Spielberg's The War of the Worlds. 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 Hopeless War 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 (including Ray's daughter -and there is no way in hell that he will let him risk that).
- The Wild Bunch: The opening heist has one member of the titular Bunch (appropiately called 'Crazy Lee') 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 Leave No Man Behind, but very much do not give a damn about him (and the fact that Thornton's posse killed him).
- Goke from The Zombie Diaries, but he may have always been nuts.
- Craig Toomy in Stephen King's The Langoliers. He's already bubbling on the verge of a breakdown before the story heats up, but he's pushed over the edge by the maddening revelation that the stale-time-eating Langoliers he heard stories about in his youth are real.
- Tedrin in Eden Green 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.
- River is the Psycho Party Member in Firefly. After her 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 concern is increased by Serenity when they find out that River was intended to be an elite assassin.
- Despite it being about a group of survivors under very stressful conditions, Lost averted this. Though Michael came so close in early season 2.
- Happens twice in The Pacific. 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 another replacement having better luck fitting in with the others, Pvt. Hamm, is shot and killed by the Japanese responding to Peck's breakdown. Truth in Television.
- Ronald Speirs in Band of Brothers has this vibe going, and actually makes it work for him. His comrades speculate whether he's really as psychotic as the stories make him, or if it's all just an act. As with the above, this is also Truth in Television, 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 popular among the men of Easy Company.
- Mick "Heat Wave" Rory in Legends of Tomorrow has never claimed to be anything more than an arsonist and a thief. He loves to eat, drink, fight, and steal... Oh, and set things on fire. Despite all this, he manages to become a valued member of the team.
- On Orphan Black, after joining Clone Club, Helena is decidedly Reformed, but Not Tamed.
- The Killing Point: Mr. Rabbit, the most violent of the five hostage takers, is a Shell-Shocked Veteran who never really left Iraq. He's constantly suggesting to just execute the hostages, and after he takes over from Mr. Wolf and starts lining them up, the Sergeant is forced to put him down.
- It's Always Sunny in Philadelphia: Charlie deliberately chooses to be this, dubbing himself the "wild card" of the gang and often doing things that make no sense (and that actively sabotage whatever the gang is trying to do) just for the sake of being the wild card. It's gotten to the point that the others have gone into a plan already expecting and preparing for Charlie to have cut the brakes of the car they're all in.
- In Bat Boy: The Musical, 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.
- In Star Wars: Republic Commando 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.
- Anders of Dragon Age II, due to being a demonically possessed Well-Intentioned Extremist or rather, a spirit of justice that got corrupted into vengeance by Anders's hatred toward the Templars.
- In a reference to Ash from Alien, above, Darth in Cube's chapter of Live A Live. He turns out to be not only a good guy, but one of only two human survivors.
- While a number of the hunters in 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 Hive Mind 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 Grand Theft Auto V. 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 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. 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.
- Although it's hard to say for sure under all the Unreliable Narrator issues and Mind Screw, Maria in Umineko: When They Cry 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 kills (or at least appears to kill) her mother over and over and over in the meta-world.
- The th Man of Nine Hours, Nine Persons, Nine Doors 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 Ace, panicking after The Reveal, and Clover, who goes Ax-Crazy after her Sanity Slippage of losing her brother.
- Although he literally fits the trope name, Belkar of The Order of the Stick is usually the Token Evil Teammate rather than this trope. As is frequently noted in-story, this is because of Roy's ability to keep him in line. When Roy dies and 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 the Oracle, whose help the party needed, it's the last straw.
- Porkfry is/was this apparently for the Penny Arcade crew, according to 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.
- The list of Things Mr. Welch Is No Longer Allowed to Do in an RPG contains some rather terrifying insights on the mind of the Psycho Party Member.
1683. 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.