God and Morality

This is a question for theists. For the record, I don't beleive in God and I don't believe God created the world. But I am interested in the beliefs of people who do believe in God.

If it is wrong to do X - X being some human action, such as killing - what makes it so?

If God says that it is wrong to kill, is this what makes it wrong? Is it wrong to kill purely because God says so? Is God's word the ultimate source of morality?

Or is it a fact of the world that it is wrong to kill, and does God say that it is wrong to kill because he is telling is about a fact that alrteady exists before he "opens his mouth"? Perhaps when God created the world, he created it in such a way that it was true of the world that it is wrong to kill, perhaps because he created a world in which killing causes suffering. So when God says "it is wrong to kill", is he just reporting a fact that he already created when he made the world?

Are there any alternatives?