If you could actually prove that God existed, that might make an interesting case. As it is, you are merely handwaving a mythical creature in an effort to give the appearence of substance to what are, in fact, fanciful arbitrary value judgements.
I cannot prove God exist, nor can you prove He does not. Therefore we weigh the virtues of the ideologies based on those two assumptions. The one that works is given credence by proxy.
Your very statement ”…to what are, in fact, fanciful arbitrary value judgments” proves my logic. Without an Absolute Moral Authority all morals and values are “fanciful” and “arbitrary”.
So how can you say one man is right and one is wrong? How can you build a society on that? A society with rules and regulations, one with laws against such actions as murder.
You cannot, unless you choose to believe that somehow one man has more rights than another or that a majority can force compliance on penalty of death. And of course neither is “moral”, just what is choosen.
At the very foundation of this country is the concept of all men being created equal and endowed with certain inalienable rights. Those rights are inalienable because no man can rightfully take what God has given you. Remove that and you have men ruling men, with no right and no wrong.
Can you tell me how Hitler was not morally justified in killing 3m Jews and 6m Christians, Gypsies and Gays? What made him wrong.