It is tiresome that two and a half thousand years since the Euthyphro people are still asking questions like this, as though morality is somehow dependent on God's will.
Does God will us to do X because it is moral, or is it moral to do X because God wills it? If the latter, then we clearly get the answer that anything could be moral dependent on the whims of the deity. We also get the question of why God's mere willing us to do X makes X moral - what are the causal relations? If the former, then doing X would clearly still be moral whether or not God willed it, for reasons entirely independent of the fact that he willed it, and in fact he wouldn't even need to exist for doing X to still be moral.