Atheism and Ethics

[Sheldon]

In a very boring and completely non novel way.

How would you respond to a Divine Command Theorist who says that God could mass kill a whole bunch of humans and it wouldn't be morally wrong because what's morally right is whatever God wills? How would you argue for your own definition of moral goodness?

If the morality of his concept of a deity, falls short of the morality of an evolved ape, why would I care what he claims that deity thinks or wants. That's assuming he could demonstrate in any objective way that this deity exists of course, if he can't then he might as well be describing the moral views of villain from a comic book.