I don't play online much, so I don't often get accused of that, but the general rule seems to be that some people just want an excuse to be angry. Their self-entitlement overpowers all of their other emotions, so they must be the one getting every kill. So if you kill someone, anyone, you are a kill stealing ******. In their opinion.
Semi-related, if I summon assassin recruits in Assassins' Creed: Revelations, and during the fight am in the process of sync-killing someone, my recruits have the tendency to push me aside to kill the enemy the rest of the way dead.
Kinda like: "No, no, Mentor, I've got this one, you hang back and just watch."
It gets a tad annoying. I CAN DO IT MYSELF GUYS.