1) When at a hardcore/metalcoreshow someone hardcore dances along the the boarder of the mosh pit with the intent of hitting the crowd. Crowd killers apparently believe that it is not hardcore to not participate in dancing when at hardcore shows. Ironically crowd killer toughness is questionable (it would be socially unnacceptable for a member of the crowd who hasn't been hardcore dancing to begin throwing kicks, punches, ect. in defense when approached by crowd killers) as they frequently hit unsuspecting crowd members who (if aware of the strike) in the context of one on one (in some cases) could stand their ground and drop that sucka. Crowd killers (as they can be called?) often target who they plan to hit and typically go after "emo kids" and/or "scene kids".
2) When a pseudo tough-guy with an ego complex punches a member of the crowd in the face (often as a sucka-punch)because it makes him feel like a man.
Billy: During the breakdown that jock hit that unsuspecting hardcore kid so hard I could hear the pop over the intensely loud chugga chuggas.
Suzie: Sweet crowd kill?
Billy: Yah. Johnny weightlifter wasn't fucking around.