Here's another chicken and the egg type question.
Are things wrong because God says so, or does God say things are wrong because they are?
If they are wrong only because God says so, isn't the a little arbitrary? Suppose God had never said that killing was wrong. Would that make it ok? And if God says things are wrong only because they are, then what good is religion? Presumably, even an atheist could figure out the difference between right and wrong without relying on God.