Why 'should' atheists be moral?

[Esquilax]

For those of you that refer to evolution as the basis for morality, do you realize that doing so reintroduces teleology into the process? If evolution leaves us with a human nature and acting contrary to it is not in the best interest of humanity, then “best interest” introduces intentionality, or desired ends, into the evolutionary process, at least where humans are concerned. That’s not a problem for believers but it is for those who think evolution is a wholly undirected process.

Except that you're wrong; no intentionality is required, as the "best interest" in this case is merely what allowed us to survive. Acting contrary to our moral nature isn't against our interest because of some externally derived intention, but because it is a detriment to the survival of the individuals that did so. In the end, those of us who act in accordance with current human nature survived at greater rates, while those that did not tended to die.

You know, natural selection? One of the main mechanisms for evolutionary change, which is entirely undirected- barring direction by wholly natural means- but easily accounts for what you're talking about? I find it hard to believe that you understand how evolution works, without also knowing about natural selection.