If there is any question at all if I'm going to make it to a tag I don't TK. If my team member had an obstructed view of the kill or I'm a medium to long distance away, absolutely it's ok for them to make sure we get the tag.
But when we're both looking at the same guy, both right next to him and I clearly kill him and then they specifically rush to steal my tag, yes, I'll shoot them with a few exceptions. I won't hold a grudge, if I kill them it's going to happen instantly after they steal the tag, if I wait more then a few seconds I let it go an move on.
Just think for a moment how many tags you yourself steal.
Maybe one every few games (accidently).
I am one of the few that will leave a tag if I know it's not mine and I see the person that earned it coming for it.
I can't tell you how irritating it is when you get the scrubs on your team that can't get any kills whatsoever so keep leeching all my tags and get killed immediately afterwards. If you are going to steal my tags, please at least live long enough to at least pull out a UAV.
When I read about the evils of drinking, I gave up reading.