last hit? Why?
It is a pain when people on your own team ks all the time. And how do they get away with it? By waiting until the enemy is just about dead, and then jump in with a high burst and kill the enemy.
I believe that behavior is toxic to the game. I don't know why Rivens always seem to want to roam into my lane, and take the kills, but it stinks. The same is true for a lot of assassins.
It doesn't have to be that way ... just give the kill to the champion that did the most damage, not the one that got an auto attack at the last second (or a skill shot, or anything else). Make ks'ing impossible because you don't get the kill unless you've done the damage. Toxicity of assassins removed. They aren't going to be interested in playing for themselves any more, but just playing for the team.