Lock them up in a real bad place for life, one that makes them wish for a death penalty. Problem solved.
HA HA! I love your solution, I agree to you, do not kill them, because only God has the authority to take away ones life, but instead, lock them in a place that is worse than prison, or put them in a society that has a bad surroundings, put them all in one place and let them kill each other. Atleast your'e not the one who killed them. so Clean so Good.