The problem in that is that you are not increasing the kill variable. If 'kill' is by default 0 you would be now setting the leaderboard value to 0+1. As the kill variable is not increased, you are always setting the value to 0+1.
You will need to increase the variable, in actions:
Set kill = (kill + 1)
Leaderboard - Change the value for (Owner of(Killing Unit)) in (Last Created Leaderboard) to kill