generally I tell my team how it is, example:
this one guy I was playing with wouldn't get the fuck out of the way at our spawn (MAG, Raven A spawn) so I knifed at him (one hit kill) only to have his friend do it before me and move in the way of my knife, I knifed him, and he wouldn't let it go and proceeded to follow me around killing me.
example 2:
anyone who doesn't do what they are supposed to in a tactical/ team based game.
also, if someone kills me first, I make it my personal mission to make sure they don't survive for more than 15 seconds at a time.