So how many are they allowed to kill? Zero? The same number? Double? Where's the line?
Overresponse can be a great way to end wars. See historical examples such as the USA nuking Japan, the german blitzkreig, and even Ghengis khan completely wiping cities off maps.
Even on a smaller scale, like if you want someone to stop punching you in the hallway at school, beating the shit out of them frequently works to end the behaviour.