Team Killer

"Fact-The key to any sucessful cooperative test is trust. And as our data clearly shows, humans cannot be trusted."

—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 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 demonstrate how evil someone is. Heroes normally don't team-kill, or at least believe that they shouldn't, because they depend on the Power of Friendship to bring them through things that would destroy less unified groups.

Played for Drama, a Team Killer is usually a reprehensible person, who everyone despises as a traitor. Team Killing is often a Moral Event Horizon. Alternately, a Team Killer might be trying to prevent the Team-Killed from doing something even worse. That said, a pre-emptive strike to stop a party member from doing something despicable isn't necessarily less dramatic, or less tragic. Occasionally Team-killing is depicted as Necessarily Evil, but even then it's nearly always morally ambiguous at best.

Played for Laughs, a Team Killer is a comic sociopath taken to its logical extreme, and chances are pretty good that the Team-Killed deserved it.

The term originates in online multiplayer First-Person Shooter 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 Bad Boss; see You Have Failed Me... and You Have Outlived Your Usefulness for elaboration. In large amounts, it's We Have Reserves.

See Betrayal Tropes 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 Player Killing.

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.

Examples of Team Killer include:

Anime and Manga

Both Guts and Griffith have done this in Berserk.
- Guts 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 sacrificing his entire goddamned army to become the fifth member of the Godhand.
Lance from Rave Master kills Bis, one of his underlings, apparently because he was taking too long to kill an enemy that Lance wanted a turn with.
Vegeta kills Nappa, his fellow Saiyan, when the latter is defeated by Goku during the Saiyan Saga of Dragonball Z.
In Robotech, Khyron the Backstabber earns his title not from Chronic Backstabbing Disorder, but because he tends to shoot his own men when they run or when they're in his way.
In one of the past timelines in Puella Magi Madoka Magica episode 10, Mami killed Kyoko in fear that they would eventually turn into witches like Sayaka. Madoka killed her in return before she could cause any further deaths.
Girghe in Broken Blade had no issues backstabbing his own team mates for no reason whatsoever.
Yeon from Tower of God is a fire mage who 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, Hilarity Ensues.
Sasuke had already defected from Konoha before he tried to kill Naruto, so I'm not sure if that counts, but Karin was his teammate when he decided that she had outlived her usefulness.
- Mizuki, the first antagonist of the series, was revealed to be this when it was shown that he killed his teammates during mission for being weak.
  - Naruto himself was dangerously close to being this. Twice. And he didn't even know about it.

Live Action TV

One of the ongoing subplots in Firefly is Jayne's willingness to sell out the team, specifically River and Simon. The subplot is resolved when Mal finds out about Jayne's attempt to turn in the Tams to the Feds on Ariel and almost throws Jayne out the airlock as the ship is taking off for it.
Tragically, repeatedly, in the 2004 Battlestar Galactica due to Cylon infiltrators.
On NCIS, poor Special Agent Lee, a rare semi-sympathetic version, was forced to kill Langer after a loved one was kidnapped.
The previous leader of Torchwood Three killed his entire team and then himself, leaving Jack Harkness to rebuild the team.

Literature

Judas. So infamous that his name is now a synonym for "backstabbing traitor."
Lijah Cuu from [[[Gaunt's Ghosts]] kills troopers Bragg and Meryl and tries to murder Saint Sabbatine herself before being brought down.

Tabletop Games

Munchkins.
Most factions in Warhammer 40,000 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:

Video Games

There are several points in Dragon Age where you can kill party members (or party members can kill you) as a consequence of "ethical disagreements."
Pick any team-based multiplayer game where Friendly Fireproof is not enabled. Griefers are often team-killers.
In Jade Empire, the player character can do this to about half of his party.
Dissidia 012 has Kain playing the Well-Intentioned Extremist variant of this. He also convinces Warrior of Light to join in on the plan, and they're both chewed out quite extensively by Lightning when the betrayal is revealed. Kain has really good reasons for doing so, however; Golbez tipped him off that if the manikins defeat any of the warriors during the war, they'll be Killed Off for Real, as the gods will not be able to revive them. This is why the likes of Lightning, Kain, Vaan, Laguna, Tifa, and Yuna are not present in the first game; they died ensuring that the Rift would be closed, preventing a huge influx of manikins into the cycle of war.
In Metroid: 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 went 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 James Pierce, the communications expert of the squad.
In NieR, the final ending provide you the choice, either kill Kaine putting her out of her misery or give up your own life to get rid of the Shade possessing her.

Web Comics

Black Mage from 8-Bit Theater. Sometimes Red Mage or Thief, but mostly Black Mage.
Referenced in Penny Arcade, when the Twelve Apostles are playing Halo.

Peter: Christ, Judas is team killing again.
Christ: Judas, don't be a dick.

No actual team kills in The Order of the Stick, but Vaarsuvius and Belkar have come close on occasion.

Roy: Dammit, Vaarsuvius, I've told you before, we do NOT blow up members of our party!

Web Original

In Red vs. Blue, 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.
- Not just Church. 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.

Western Animation

Part of Scott's strategy on Total Drama Island. He seems more interested in eliminating his own teammates than the other team.