Friendly Fire

Team killing is a slang term used for a player intentionally killing a team-mate; the term may also be applied to accidental friendly-fire. Team killing is been popular among many and dates back to Call of Duty. Today, Team killers are often found in Modern Warfare 2 online matches, in Hardcore matches where friendly fire is enabled.

Team killing is often done as a form of revenge for a "kill steal", to reciprocate a team kill, or for any number of reasons including "griefing" (to deliberately harass other players).

Call of Duty
Servers on the original online multiplayer could or could not have friendly fire turned on. At the time there was four options. "Disabled" in which you could not kill teammates. "Enabled" which meant you could kill teammates. "Reflect" which is popular since team-killers will actually kill themselves, and "Shared" which is a combination of Enabled and Reflect.

Call of Duty 2
Call of Duty 2 has the same settings as the original and behaves very similarly.

Call of Duty 3
In Call of Duty 3, friendly-fire can now be responded to; if a person is team-killed they will now get the option to either punish or forgive the player. Punishing takes away 3 points from a player's score, often resulting in negative scores on the scoring boards, such as -15 for five team kills and no legitimate kills. If forgiven, the team-kill will have no effect, as it might have been by accident; however, it is up to the killed player to forgive or punish, so an accidental kill may result in a Punishment anyway. Players are warned that they may be kicked from a match at 7 team kills, and kicked after 9.

Call of Duty 4: Modern Warfare
The Call of Duty 4: Modern Warfare manages team kills more simplistically. And the normal Team Deathmatch has no Friendly Fire enabled, unless done so on a Private Match. The only friendly fire game modes are "Hardcore Team Deathmatch", "Hardcore Headquarters" and "Hardcore Search and Destroy". In Hardcore Team Deathmatch, the player has no choice over whether or not they are punished or forgiven. Often, Team Killers will enter Hardcore Search and Destroy with RPG-7x2, Sonic Boom, Martyrdom. At the start of a match, they take out the RPG-7 and blast the ground, killing everyone around them. Hardcore Search and Destroy is one of the most popular Team-killer places in the game only for the reason it has Friendly Fire enabled and that you don't respawn. Upon two team kills the player will automatically die and every other team kill thereafter.

Call of Duty: World at War
Many aspects of team-killing are the same. Hardcore mode is still the only one with friendly fire enabled. Team-killers enter Hardcore playlists armed with Bazookas, grenades, Satchel Charges, and any gun they carry and wreak havoc on teammates. Upon killing three teammates the player will be kicked unless he/she is the host. If a player kills a teammate within a few seconds after a match starts, they will die as well and will lose 1 point for a suicide as well as the 10 points for team-killing.

Call of Duty: Modern Warfare 2
Team killing is still possible in Modern Warfare 2 in the hardcore game modes. However, there is a hardcore game mode that prevents players from killing their teammates by reflecting the bullets and killing the player. In private matches, players will NEVER get penalized for friendly fire. They are free to constantly teamkill with no limits. The most common team killing in this game is for Care Package and/or Emergency Airdrop crates, as players will wait for the package to come down and kill the owner if there is a high killstreak inside, such as an AC-130 or EMP.