But we're all in the same defintional boat. You might call kindness good but have no way to knowing this to be true outside a definition which says "kindness is good".
I think the point of man-made morality is not what necessarily what it says, since that can vary between individuals and cultures, but how it is arrived at. It is not simply a case of "kindness is good" but is instead a recognition of how kindness can affect our lives for the better, and the lives of those close to us. Similarly we learn from experience how being "unkind" can hurt emotionally and/or physically, and we can recognise that in others. Yes it could be argued a very simplistic take on morality, the idea of empathy and not doing to others what you wouldn't want done to you, but it works. This is why we can say that slavery, rape, or genocide are fundamentally wrong.
But since God does not share in the experiences which have shaped our moral systems, how does he arrive at what is "good"?
Which means the possiblity exists for me killing my family to be Gods request of me and Hitlers killing of many not to be Gods request of him (despite his claims)
But why do you automatically assume that God wasn't instructing Hitler, as part of a grander plan. Maybe God hardened Hitlers heart like he did Pharaohs, to make him commit those atrocities. After all you accept the genocide of the Midianites was necessary, even "good" because God ordained it, so why not the holocaust?