We could all prey on the weak, killing each other at will. But then, societal life would be impossible. I've made a decision to agree to live by and uphold certain societal norms, so we can work to the benefit of all mankind. Its very much a golden-rule type situation.
Oh, and you say that "most" people subscribe to this belief. What about those who don't?
They cannot coexist in a civil society. So they can either go to prison, or they can have fun in a chaotic free-for-all like Somalia.
Who are you to judge them? You're just an "ape" like they are.
No, I'm a man. And this country is a community of men and women, so we get to set the rules.
Ok, who determines these societal norms? An on the golden rule, if you're an atheist, what are the consequences of not observing it? There are none, really.