Alright, some good and bad points made.
Let's throw a twist on it. So basically a demon offers you to kill people who will be going to Hell regardless (majors like murderers, and finger cutting rapist), not minors like say for instance a person who worships satan, or doesn't want to be bothered with God.
The demon made sure you didn't get hurt, and you never rested, you went on a spree 24/7. The downfall of the deal is that you'd be going to Hell after the deed is done, the good news is that you whiped out over 90% and one of the urderers you killed saved your families life (hence one of the killers were going to kill your family).
Would you accept the deal? Or no?