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1.
In online multiplayer games, when somebody else kills the enemy you are fighting. This often results in you earning less experience points than you normally would for killing that enemy because two people were attacking it.

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!"
by Clockwerk January 06, 2008
 
2.
Noun, when you're about to kill an enemy in an online game, and another player comes out of nowhere and kills them instead, in one or two shots, usually a n00b.
Shit dude, I was about to kill Matt-O when this douche jumped up nd got a headshot!! Killsteal.
by Matt-0 June 17, 2009
 
3.
Attacking a mob in a MMORPGthat has already been engaged in combat. Usually for the purpose of obtaining looting rights over another individual or group.

Often a tactic used by a griefer to annoy other players.
You tried to kill-steal Peter Lee again, but failed miserably.

That guild of griefers keeps trying to kill-steal when we spawn mercs.
by camb November 18, 2003
 
4.
When you playing an online game that involves killing, and right when your about to kill someone, someone jumps out and kills him first
player 1: OHHH IMA KILL YOU
player 2: NOOOO
player 3: hahahah KILL STEAL!!
player 1: WTF U HAXZOR ROFLCOPTER ROFLCAKES!!!
by Slyslutter July 20, 2006
 
5.
guy who plays halo and hangs around watching there teammates mostly die, and then swoops in for the kill.
oh my god, collin is such a killsteal.
by gumpwumper February 04, 2009