Only noobs do this, and is considered a gamer's equivalent of a war crime among many.
"Dude, I was fighting a (insert enemy) and this noob came out of nowhere and stole my kill."
_ _ _
"Don't kill steal, asshole!"
Often a tactic used by a griefer to annoy other players.
That guild of griefers keeps trying to kill-steal when we spawn mercs.
player 2: NOOOO
player 3: hahahah KILL STEAL!!
player 1: WTF U HAXZOR ROFLCOPTER ROFLCAKES!!!