Reductios and the argument from evil

This links to a bunch of posts I did on the topic of the argument from evil as a reductio. What happens when someone who doesn't believe in moral objectivity defends the argument from evil?

The problem arises when the argument requires are fairly finely tuned moral claim. Someone might say "If God is good, then he can't allow gratuitous suffering. The theist answers "Why? What's wrong with permitting gratuitous suffering?" The atheist can't say that it's objectively wrong for God to do that, so he has to argue that the theist believes that it is wrong for God to do that? The atheist then has to work from inside the moral belief system of the person with whom he is arguing. But moral belief-systems amongst theists, and even Christians differ from one another.

Calvinists think that God is justified in creating people for the sake of reprobating them in hell forever, for the sake of God's own glory. I happen to disagree with them, but I believe in objective moral values. How can a subjectivist argue with them?