God does not do the trick nicely at all, i.m.o. Killing a man because he made a graven image would be unethical, as is the militaristic conquest of other peoples' lands, and no amount of other-worldly power could make it otherwise. Similarly, there's nothing profoundly ethical about not doing work on a particular day of the week--even if an almighty power orders it.
Here's a simple outline of how morality can be accounted for through a naturalistic worldview: By nature, and by definition, we want to be happy and avoid pain and suffering (people with normal brains, that is). Ethics is just the quest to establish guidelines of behavior that will promote happiness and mitigate suffering for everyone. It is not necessarily an easy task, given how complicated happiness and suffering have turned out to be via the study of such subjects as psychology, anthropology, economics, political science, all medicine and the natural sciences and especially history. (Despite popular misconceptions, doing lots of drugs and hoarding money and backstabbing will likely make you less happy, not more.) But it is possible in principle to establish things as objectively more or less ethical (or neither) insofar as they objectively make us more or less happy. And I think that there are a lot of things which we know are unethical and which *seem* at first glance to be advantageous to one's happiness; and there are similarly many things that are ethical which *seem* to cause pain and suffering. But I suspect that in all of these cases, fully accounting for all the likely consequences / realistic constraints shows that those things which we think of as unethical will make us, on the whole, less happy. And in those cases where our biological or cultural intuitions about ethics cannot be resolved with the best interests of human happiness--for example, every culture reviles incest, but what about a special case where the couple is infertile? --human happiness ought to take precedent, and the old intuition rejected as a misguided taboo.
And it is possible, through education and thoughtful reflection, to derive greater happiness from the happiness of those around you. That's a desirable situation for everyone: your happiness makes me happier, and my happiness makes you happier. Seeking out such positive-feedback loops with others is both rational and enjoyable. Seeking positive expressions (e.g. team sports) for some of our fundamental negative impulses (tribalism) is also rational and enjoyable.
Of course, there are people who don't derive any satisfaction from the happiness of those around them, and don't feel pain when they inflict it on others. We call them psychopaths. Their brains are abnormal, and why Heaven would create them without the mental faculties for compassion and then command them to have compassion is for the theologian to explain. For the scientist and the ethicist, it is enough to note that such people exist, and that they are not "evil" (at least in the naive sense of the word). Although the pious throughout the centuries righteously took revenge on such people, today we think it enough protect them from themselves and others.
And of course, there are situations where (virtually) everyone will resort to violence, a mob mentality, stealing, etc. Those tend to be situations where resources are scarce or the sense of fairness of the masses has been outraged. What we would like to do, then, is avoid such situations by doing our best to provide everyone with basic "human rights".
