According to the common theistic view of moral ontology, facts about moral value reduce to facts about what God is like. This is an a posteriori property identity claim, that the property x is good is literally identical to the property x is like God's essential nature. As such God is not just one good thing among many, but rather he is the very standard to which all good things measure up. He is, according to this view, what Plato called the Good. Rather than explaining evil, theists typically explain it away. The common analogy is that, just as darkness is the absence of light and cold is the absence of heat, evil is merely the absence of good; the absence of any likeness to God. Just like a hole in your shirt, evil technically doesn't exist. It's not a thing in of itself. Even yet, just as a hole in your shirt will leave you cold, evil can still negatively affect you. Having an absence of God or his likeness in your life will, as it's thought, lead you to ruin.
The motivation for this view is that it explains, or best explains, the nature of morality in such a way that preserves our common ethical opinions. Since God is necessarily loving and kind and just, we get, as a result, that it's good to be loving and kind and just. But there are other essential characteristics of God that don't seem to fit into this paradigm. For example, God is necessarily prudent in that he doesn't thwart his own goals, but rather acts efficaciously to bring about his divine will. God is necessarily rational, in that he doesn't accept or hold propositional attitudes that go against the dictates of reason. And God is necessarily immutable, in that he does not behave inconsistently or arbitrarily from one day to the other. It would be rather absurd to think someone is failing to live up to the moral standard by being irrational, or acting imprudently. There might be a sense in which someone who believes contradictions is doing something objectively wrong, but this isn't a moral wrong. Believing contradictions makes one guilty of foolishness or irrationality, but not of moral wrongdoing. So, it seems, this account of moral ontology must fail.
There are further problems with the attempted explanation of evil. If evil is merely an absence of good, then good and evil must partition all things. Something must either be good, or be in the absence of good; there isn't any third option. But of course this is absurd, the vast majority of actions we perform are neither morally good nor evil. Every choice would become a moral choice, and every action would be morally significant. So again, it seems this account of moral ontology must fail.