You lose me when you state this. What God are you talking about? Just the Christian God? Can Hindus, Buddists, not be moral? When an agnostic refrains from murder is he not being moral?
If there's no God, then there's no morality.
Here is how I separate it out. Murder: would you prefer to be a) the victim; b) the murderer; or c) neither. No appeal to God is needed. Universally, without regard to culture, only a miniscule fraction would entertain option a. Murder is wrong, QED.
Depends on which man is defining "moral" now doesn't it?
You assume murder to be an "immoral" act. Who says that is true?