Middle Path –
You bring up a real difficulty when you said, “If he doesn’t believe in G-d, he will have no problem stealing, murdering, etc., because the only thing preventing him from doing such things is that ‘G-d said not to.'” As strange as it may be to say it, this is in fact the reality of the situation. An individual who does not believe in G-d lacks any real imperative to be what we would deem a “good” person. When I say lacks any real imperative, I mean an absolute imperative. IE he may not murder, steal, cheat, etc. because of fear of getting caught, being ostracized by society, or because he personally feels it to be wrong. But, these are all relative reasons. It is only through H’ that we have an absolute right or wrong and with that a real, external and immutable, reason and chiuv to perform mitzvahs and avoid aveiras. That being the case, if one were to remove G-d from the situation (so to speak), there’s no real reason to help another, honor parents, etc. as well as not to murder, steal, cheat, etc. Granted, society may very well disagree with what an individual does, but once again society’s norms are relative norms (think Nazi Germany Yemach Shmo). One could argue, “Derech Eretz Kadma l’Torah,” but that still only applies in a framework where there is G-d. Removing G-d from the situation also removes all moral imperatives. Looking forward to your response.