This argument implies that morality is no more than God arbitrarily assigning values to actions. After all, an action is not good because God believes it to be good; it is merely good because he wills it, a seemingly arbitrary determination. According to this view, even if the Bible encouraged murder, Christians would be expected to blindly follow "God's will."
Moreover, this argument implies that the Bible explicitly defines right and wrong in every situation, making moral decisions clear-cut and contradiction-free. This is patently untrue. Even if a person attempts to follow the God's word to the letter, moral inconsistencies will abound. For example, two of the most well-known commandments are "thou shalt not lie" and "thou shalt not kill;" but would it be acceptable to lie to save a life? If so, is there a hierarchy of sins? How do we define "lie" and "kill" in the first place? For such questions, the Bible suggests no answer.
Finally, if God is not all-knowing or all-loving, we cannot trust his will to reflect true morality. If God does not exist at all, the entire argument is irrelevant.