Psycho Party Member

"''"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.'""

- Dane Cook

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.

Could be a result of a maddening revelation.

Contrast with Token Evil Teammate, Could also overlap with A House Divided, and Zombie Infectee.

Anime and Manga

 * 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.

Comic Books

 * The Walking Dead has quite a few of these.
 * Specifically  and of course the group goes through the whole should we or shouldn't we kill him debate. Because Murder Is the Best Solution.
 * Michonne, and even Rick teeters on the brink of this sometimes, and possibly
 * And of course, Carol.

Film

 * Dr. William Weir from Event Horizon.
 * Rockhound from Armageddon.
 * While he can be considered psychotic, he doesn't count. He is a trusted member of the team; and doesn't crack under pressure. It is the American astronaut who nobody knows, that is the problem. All the others have worked together for many years on the oil rig.
 * Except they didn't trust him to detonate the nuke.
 * Quentin from Cube.
 * Goke from The Zombie Diaries, but he may have always been nuts.
 * Arguably Andre from Zack Snyder's Dawn of the Dead 2004.
 * Blair from John Carpenter's The Thing. Although there's a good chance he was right to act so crazy. At first, anyway.
 * Ridley from Diary of the Dead.
 * Panic Room - Raoul. Things would really have been a lot better if Junior hadn't gotten him involved.
 * Ian from Final Destination 3.
 * Pvt. Miguel Salazar from Day of the Dead.
 * Arguably Capt. Rhodes, although he's the most practical, and the only one that makes any sense.
 * Logan as well.
 * Harlan Ogilvy from Spielberg's War of the Worlds.
 * Pinbacker from Danny Boyle's Sunshine,.
 * Xavier from Saw II.
 * Ash from Alien.
 * from Pandorum.
 * Mr. Blonde from Reservoir Dogs. A good 90% of the dialogue anyone else says of the man is how psychotic he is.
 * Waingro in Heat.

Literature

 * Arfy from Catch-22.
 * 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

Live-Action TV

 * Despite it being about a group of survivors under very stressful conditions, Lost averted this. Though Michael came so close in early season 2.

Theater

 * In Bat Boy the Musical,

Video Games
"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."
 * Although it's hard to say for sure under all the Unreliable Narrator issues and Mind Screw, Maria in Umineko no Naku Koro ni 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
 * The [9]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
 * In Republic Commando Scorch worries that Sev may be one of these in one of the loading screens:


 * Anders of Dragon Age II, due to being a demonically possessed Well-Intentioned Extremist.
 * In a reference to Ash from Alien, above, Darth in Cube's chapter of Live a Live.

Webcomics

 * 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, 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 , it's the last straw.
 * Vaarsuvius fits as well, some of the time.

Web Original

 * Everyman HYBRID brings you Evan.