Yes, I have killed teammates before, although I had my reasons. I am not sure if the OP would "count" them or not.
So, here they are:
1. The teammate kills me first, in a manner which is clearly not accidental. i.e. killing me for a specific weapon I have in my possesion. In these cases, I will spend the entire rest of the game hunting this person in order to kill them over and over, or to weaken them, and allow the enemy team to kill them over and over.
2. Accidental kills, in which by an unfortunate circumstance, I inadvertantly kill one or several teammates.
3. The teammate is consciously trying to "foul up" the game. i.e. shooting at teammates without killing them, intentionally going the wrong way in a game of capture the flag
4. Killing friends, I do this only when our team has a clear and obvious lead, in which we could not possibly lose, even if we wanted to. In these cases, I usually only kill my friend once, for laughs. Usually, the friend in question will kill me in return, and then both of us will continue the game in normal fashion.