A redated post from 06.
I have been arguing 1) that there are arguments from naturalism to moral subjectivism ( I have more to say about that later) and 2) moral subjectivism undercuts the argument from evil. Now I agree that if the atheist can show that his theist opponent is a moral objectivist who somehow implicitly accepts the moral premise of the argument from evil, then he would have a reductio ad absurdum argument against that theist. But once the reductio is done, the theist may not reject theism, he may just deny the moral premise of the argument from evil, in order to make his position consistent. What you actually have to show is that theism, or Christian theism entails that moral premise, but showing that would be a tall order.
It's not true that no theists defend moral subjectivism; I got the impression that Marilyn McCord Adams, a leading Christian philosopher first at UCLA and later at Yale, is a moral subjectivist of some kind. Her husband Robert Merrihew Adams (making the two of them the philosophical Adams family) is a moral objectivist, (see, a philosopher couple can disagree about a few things!) but in a paper originally published in Philosophical Review which later appeared in his book The Virtue of Faith, entitled "Must God Create the Best," argued that God did not have the obligation of creating the best of all possible worlds, or even the best world that he had the power to create. In that essay, and in another essay entitled "Existence, Self-interest, and the Problem of Evil" Adams argues " that ethical views typical of the Judeo-Christian tradition do not require the Judeo-Christian theist to accept the view (that this is the best world God could create). He must hold that this world is a good world. But he needs not maintain that it is the best of all possible worlds, or the best world that God could have made." Now if you're a moral objectivist, you can argue that this position is morally unacceptable based on a moral standard you share with Adams and other theists. If you are a subjectivist, you're out to lunch in the face of an argument like this.
William Lane Craig, in a debate with Keith Parsons, argued that since God had created the Amalekites, he had the right to have them slaughtered down to the last man, woman and child. Parsons expresssed "outrage" at this position. I don't buy Craig's theology here at all, but as a subjectivist, I would have no right to complain about it. I could not argue that Craig was being irrational in accepting this kind of a moral position. I don't know if Parsons is a subjectivist or not, but if he is, his outrage is just his personal feeling, and there is no way he can make it binding on Craig.
So I think that even though it is theoretically possible to use the argument from evil as a subjectivist, the theist has some counter-moves which look awfully difficult to stop, consistent with moral subjectivism.