Power in Numbers - 2 (or more) players who think they're skilled by ganging up on a single player, usually accompanied with sh*t-talking towards the victim if mic'd up.
Those pricks piss me off the most! You could be having a 1v1 battle and then the enemy will bring their friend in and they'll continuously kill you until you escape and get called a pussy for running. Sometimes players deserve it, like today when me and a fellow crew member were assaulted coming out of ammunation by some level 42 using his auto shotgun so we killed him and taught him a lesson for thinking he a god for killing two people un-aware.
The others that piss me off is when one of your friends invites a friend and you get told not to kill him but when he kills you for no reason it's perfectly f*cking fine.