What are you basing this on? You're making the claim as if it's fact, but haven't cited any evidence. Studies of inmates in the US, for example, disagree. Most of them are not atheists.
Prima facie - and people in prison often come to change their outlooks. If you believe in God, then you know murder is wrong. You can (of course) choose to go against God's word... But if you do not believe in God, then there is nothing holding you back...the only person you answer to is yourself...and you may in fact give yourself permission to kill...