Plantinga's argument from evil for theism

This is also from the essay "A Christian Life Partly Lived," from IVP's Philosophers who Believe.
Second, it is indeed true that suffering and evil can occasion spiritual perplexity and discouragement; and of all the antitheistic arguments, only the argument from evil deserves to be taken really seriously. But I also believe, paradoxically enough, that there is a theistic argument from evil, and it is at least as strong as the atheistic argument. (Here I can only sketch the argument on an intuitive level). What is so deeply distrubing about horrifying kkinds of evil? The most appalling kinds of evil involve human cruelty and wickedness: Stalin and Pol Pot, Hitler and his henchmen, and the thousands of small vignettes of evil that make up sucha whole. What is genuinely abhorrent is the callousness and perversion and cruelty of the concentration camp guard taking pleasure in the sufferings of others; what is really odious is taking advantage of trust (a parent or a counselor, perhaps) in order to betray and corrupt someone. What is genuine appalling, in other words, is not really human suffering as such so much as human wickedness. The wickedness strikes us as deeply perverse, wholly wrong, warranting not just quarantine and the attempt to overcome it, but blame and punishment.

But could there be any such thing as horrifying wickedness if naturalism were true? i don't see how. A naturalistic way of looking at the world, so it seems to me, has not placce for genuine moral obligation of any sort; a fortiori, then it has not place for such a category as horrifying wickedness. It is hard enough, from a naturalistic perspective, to see how it could be that we human beings can be so related to propositions (contents) that we believe them, and harder yet, as I said above, to explain how that content could enter into a causal explanation of someone's actions. But these difficulties are as nothing compared to seeing how, in a naturalistic universe, there could be such a thing as genuine and appalling wickedness. There can be such a things only if there is a way raitonal creatures are supposed to live; and the force of that normativity--its strength, so to speak--is such that the appalling and horrifying nature of its its inverse. But naturalism cannot make room for that kind of normativity; that requires a divine lawgiver, one whose very nature it is to abhor wickedness. Naturalism can perhaps accomodate foolishness and irrationality, acting contrary to what you take to be your own interests; it can't accomdate appalling wickedness. Accordingly, if you think there really is such a thing as horrifying wickedness (that our sense that there is, is not a mere illusion of some sort), and if you also think the main options are theism and naturalism, then you have a powerful theistic argument from evil.