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Players who team kill often do so for the reaction of the other players. The team will be at a disadvantage, and the team killer is usually scolded at by text chat, voice chat, or by being team killed themselves. Team killing takes away enjoyment of the game from the victims, and most game developers seek to minimize the impact of the practice. Some games will give other players the ability to kick an undesirable player, usually a team killer, from the game. Other times, games will avoid team killing by not allowing members of the same team to damage each other in the first place.
Some team killers will also take out a fellow comrade to prevent the opposing team from getting ahead. Such is the case with Call Of Duty 4, in which a player in Last Stand in a hardcore match will often ask a teammate to kill them so the enemy that downed them won't get the points.
Sometimes, team killing can be done accidentally. Usually players will be caught in an explosion of a grenade or rocket a teammate intended for an enemy. The victim may think the team kill was intentional, and will try to get the player booted, despite no intentional wrongdoing. The only way to defuse this situation is to communicate with the player about your true intention.
Team killing as Game Mechanic
In Team Fortress 2, friendly fire is turned off, yet many players attack teammates anyway. They are trying to find Spies, a class that can disguise themselves as members of the other team. A sign that a teammate is really a Spy is if they take damage from your or another teammates' attacks. The Pyro is the best at finding Spies. The flamethrower will ignite a disguised Spy, revealing him to the rest of the team.
In the Halo series, team kills are known as Betrayals, complete with an announcer noting the event. If a player in Matchmaking games causes multiple betrayals, his (or her) most recent victim is given the option to boot the player from the game. S/He can also choose to forgive the player if their betrayals were accidental. In some cases, the option to kick someone that betrayed you comes up after the first team kill, which has lead to a unique exploitation of the system; a griefer will goad other players into killing them by attacking them without killing them or by destroying the team's own vehicles. Once their trollish behaviour actually leads a team member to kill them, the griefer then uses the betrayal system to boot the innocent victim.