I think that there should be a 1 sec 'invincible' spawn thing. Maybe not at the end of rounds, but when you get revived.
As for the not killing someone when shooting them..
When you shoot, the game reloads and the next paintball is shot. The different shot rates depends on how long it takes to reload. So say you shoot, start relading, get in someone's face, try frantically to shoot but it wont let you (because its releading the next paintball), then get shot, it is because you missed the first one and are reloading, even if it's for a millisecond, that gives the other person a millisecond to shoot in. Therefore, he kills you before you load in the next paintball. Not a bug.