As far as the question of what God, “we” would say any and all God’s don’t exist. There is no “correct” religion, unless you think a bear is your God then I can’t exactly deny the existence of bears, I just wouldn’t think they created the universe and left themselves as bears lol

I’d say it’s a combination of the things you suggested as far as what stops them exactly, this person could believe that their lack of a desire to kill was God stopping them, or it could be the fear of consequences in the afterlife, but there is definitely a fear of consequences from their fellow citizens. I live with that last sentiment as well: this is all we get so I want to be “good” to all those around me, not for fear of negative consequences for me, but the consequences of my actions creating a more negative experience for those around me. If God is real and is just then he’ll judge my actions, not whether I believed he exists or not. If he does judge me for that, well I’ll take it, I wouldn’t care to be on that kind a of petty asshats good side anyway.

Either way, if I have to live in a world with religion I’m at least happy that it brings some people solace and it stops nutjobs from acting out (mostly.)