I try to avoid people who cry about something as stupid as a kill being "stolen". It usually means that they aren't team players which usually result in our loss anyway. Unfortunately, they can be hard to avoid if you can't find the right group to play with. If I'm in a game, and I do shamelessly shoot someone who is in last stand and obviously not a threat, I apologize before they even get the chance. If it's during a chaotic firefight or if I run into somebody who isn't completely dead who is about to shoot me and they still say something. I just "You'll get over it". Or sometimes "Yes, I have 7 assists, you'll get over it".