Without God murder is actually wrong based on ___________________.
its definition. "Murder is not wrong" is a self-contradicting statement, since the word murder can only mean wrongful homicide.
A trivial point to make, certainly. Far more interesting are the criteria by which homicides are deemed to be morally and legally culpable (murder) or not, and the variability we find here throughout history and ethnography is huge: there have been, and are, many societies which classify as justifiable homicide what we would call murder.
As succinctly put recently by Ingledsva -
people around the world have different ideas and laws - that are not actually based on any God - but on what the society deems lawful.
And what they deem lawful (and morally acceptable) is generally what lets them prosper.
Your own response to this very obvious point, reliable as a knee-jerk, has generally been to summon up your old ally Adolf:
If you can define reality into existence based on opinion why should Hitler not be able to do the same?
You are, I'm sure, aware of the argument fallacy called Appeal to Adverse Consequences. Your argument in the above quote boils down to "morality must be absolute and god-given, otherwise I would have no solid ground from which to call the Nazis evil" - a classic appeal to consequences. You and I no doubt share a common disgust of Nazism and its deeds. Neither of us need like the fact that that disgust is culturally rather than absolutely grounded - but not liking the fact doesn't invalidate it.
You base your view of morality on the way you wish it to be; I (and others such as, I'm guessing, Ingledsva) on how we see it to be. It operates in the world as an observable sociological and anthropological phenomenon, not as a philosophical abstraction.