So you only don't kill people because you're worried about hell? If you could get away with murdering someone and there were no god you'd do it?
The roots of morality are different for every atheist. For most of them I imagine it's more of a habit than anything else, as well as the power of social norms. For me personally, it's an outgrowth of my primary value, which is human happiness. I recognize that this is an arbitrary value, but it's one I personally hold very dear and it forms the basis of my ethics. I value the happiness of myself and others so I refrain from actions that clearly cause a net increase in unhappiness. Murder, theft, lying, etc. all fall into that category. I do believe that I could do many of those things and probably get away with it, certainly in a cosmic sense I would get away with it at the time of my death when my consciousness ceased to be, but I have no reason to do those things and I'm pretty sure that it would make the world a worse place if I did them. In general I don't want to live in a world where murder, theft, rape, or other crimes are common so I refrain from them myself. There's a dash of existentialist angst in there too. While I don't feel as strongly about it as Sartre I do think that to a certain extent my choices define what I believe to be good for every man, so I try to choose carefully.
Important point: don't confuse atheists with nihilists. There are plenty of nihilistic atheists and I imagine most nihilists are atheists, but most atheists are decidedly not nihilists.