From the book Language of the Night:
Critics have been hard on Tolkien for his "simplisticness," his division of the inhabitants of Middle Earth into the good people and the evil people. And indeed he does this, and his good people tend to be entirely good, though with endearing frailties, while his Orcs and other villains are altogether nasty. But all this is a judgment by daylight ethics, by conventional standards of virtue and vice. When you look at the story as a psychic journey, you see something quite different, and very strange. You see then a group of bright figures, each one with its black shadow. Against the Elves, the Orcs. Against Aragorn, the Black Rider. Against Gandalf, Saruman. And above all, against Frodo, Gollum. Against him--and with him.
It is truly complex, because both the figures are clearly doubled. Sam is, in part, Frodo's shadow, his "inferior" part. Gollum is two people, too, in a more direct, schizophrenic sense; he's always talking to himself, Slinker talking to Stinker, Sam calls it. Sam understands Gollum very well, though he won't admit it and won't accept Gollum as Frodo does, letting Gollum be their guide, trusting him. Frodo and Gollum are not only both hobbits; they are the same person--and Frodo knows it. Frodo and Sam are the bright side, Smeagol-Gollum the shadow side. In the end Sam and Smeagol, the lesser figures, drop away, and all that is left is Frodo and Gollum, at the end of the long quest. And it is Frodo the good who fails, who at the last moment claims the Ring of Power for himself; and it is Gollum the evil who achieves the quest, destroying the Ring, and himself with it. The Ring, the archetype of the Integrative Function, the creative-destructive, returns to the volcano, the eternal source of creation and destruction, the primal fire. When you look at it that way, can you call it a simple story? I suppose so. Oedipus Rex is a fairly simple story, too. But it is not simplistic. It is the kind of story that can be told only by one who has turned and faced his shadow and looked into the dark.
There's another important theme that runs through Tolkien which is sub-creation. The act of creation involves depositing and depleting your power into the creation. This is why Arda is known as Morgoth's ring; he deposited himself into the world, corrupting it in the process from the original design. The possible exception is that Eru maybe did not diminish himself in the process of creating Arda but I'm not sure. One way to think about it is that Eru did diminish himself via sub-creation and that would make Morgoth an aspect of Eru in the same way that the rest of the Valar and Maiar are. It could be entity and shadow-entity counterpart all the way down to the root, meaning sub-creation is really just dissociation and God is simultaneously good and evil the whole time.