Well first off a thing is right or wrong simply because God's says it. Do we christians locate the source of that rightness or wrongness in God's unchanging charector? Yes but what we know of that charector is revealational in nature and always anthropomorphic. We cannot use reason to probe the depths of God. We can faithfuly start with his revealation to us and move from there. Also we must say yes God can declare murder to be right, but he has covenantly bounded himself in the covenat of works (or creation). Remember that God can do whatever he wants because he wants to. Does he bound himself in covenants to mankind? Yes, so he will no more "change" his mind on murder being wrong than he will flood the earth again.

I do not object to appealing to his immutible charector, only that appeals alone tend to abritraraly bound God for some logical or metaphysical reason. God is no more bound by logic than he is bound by creation. Now murder is one crime that he gives his reasons for in the Nohaic covenant but he is under no obligation to give any reason whatsoever for his descissions regarding anything. That I think is the best response to the Euthryphro argument, and Ryft gave an excellant response to it as well.