Why 'should' atheists be moral?

[vincent150]

You can't just expect everyone else to sign that agreement while you go "but I'm the exception."

This is exactly what I'm asking, why should someone not let themselves be the exception? Yes, it would ruin society if everyone said this but if you know everybody else isn't going to say this, which is how it is in out society, you can benefit from being the exception. Religious people would say they wouldn't do this because of God, what is the reason an atheist can give for not doing it.