Faith, atheism, and sinning

A book I'm reading right now just reminded me of something I was told a few years ago: for some religious people, the main reason why they don't engage in acts that are generally considered to be "sins" (e.g. killing, stealing, adultery) is because their god doesn't want them to. Therefore, they find it particularly disturbing that some people are atheists, because they simply cannot see any reason why a person without a god wouldn't be killing, stealing, committing adultery etc. whenever the urge strikes them.

I can't speak for all the atheists everywhere, but for myself personally, the reason why I don't go around killing/stealing/committing adultery is that I can put myself in the potential victim's shoes. I can imagine how much it would suck to be murdered when you still want to be alive, or to have something that you need or something that's important to you stolen, or to be cheated on by someone you've promised to love forever. It never even crosses my mind, not even in vengeful fantasy, to engage in these acts because no satisfaction I could get from it could possibly be worth the pain it would cause the other person.

So I find it kind of creepy that for some people the consequences of their actions on others are a minor factor (if a factor at all) in their choices of behaviour, and the main thing standing between them and murder is their god. However, I'm sure these people find it equally creepy that god plays no role in my decision-making, and the only thing standing between me and murder is that I think it would hurt the other person too much.

This does raise the question: suppose a religious person with this particular attitude loses their faith for whatever reason. Do they still have a reason not to murder? Or do they develop the habit of prioritizing the effects of their actions on other people? Or is the law enough to stop them?