Friendly Fire

Team-killing is a slang term used for a team member killing his or her own team mate. Team-killing has been popular among many and has dated back to the first call of duty game on the PC. Today, Team-killers lurk around in Call of Duty online matches, most notably Hardcore matches where friendly fire is enabled.

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
In Call of Duty 2, it's pretty much the same thing. No differences.

Call of Duty 3
In the 3rd installment, Team Killing is known to bring consequences. If a person is team-killed they will now get the option to either punish or forgive the player. Punishing takes away a player's point and this is why you might see team-killers with negative scores on the scoring boards (Ex: -15) If forgiven, the team-kill will have no effect, as it might have been on accident.

Call of Duty 4: Modern Warfare
The 4th installment manages online incredibly well. 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" and "Hardcore Search & Destroy" in HTD, the player has no choice over whether or not they are punished or forgiven. In Hardcore Search & Destroy, people usually give themselves RPG-7 x2, Sonic Boom, & Martyrdom. At the start of a match, they take out the RPG and blast the ground, killing everyone around them. Hardcore Search & 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.