Why 'should' atheists be moral?

[Angrboda]

I agree we are the way we are because we evolved as social creatures but now we are intelligent enough to realise that why do we not go back to every man himself.

Because that would be stupid, as you've conceded elsewhere when you acknowledged that a society where we don't do that is better than one where we do. Broken individuals may not have sufficiently developed moral sentiments of fairness and reciprocity, senses of guilt and shame, or sufficient empathy to refrain from preying on their fellow humans; but the majority do.

I believe he's asking a higher level question. Where does the moral dimension of moral questions come from in an atheist world? I may choose to eat that extra slice of pie, and I shouldn't because I don't like the consequences of eating it, but consequences alone don't make the should of not eating a piece of pie into a moral 'should'. No matter the consequences of eating that piece of pie, it doesn't become a matter for morals. Now if I choose to steal something, that shouldn't has a moral dimension that eating the pie does not, even though I may suffer just as much from both. The question I think he's asking is where does this 'moral dimension' come from?