If an NPC would steal a kill, then I would consider it to be part of the game. If a player steals one random kill, I would also consider it to be part of a game. But when a player stalks you and tries to steal every kill it can from you, then there is a human being behind that keyboard trying to ruin my gaming experience.
When they single you out and follow you around, then that becomes harassment and is punishable as such.