First of all we are led in the imaginary world of Middle-earth, at the very beginning of man's history. (this gives me a feeling of "primitivism", the moment when the good and evil began to shape). Middle-earth is kind of dreamlike: a world of magic, fear and beauty where strange creatures work for the powers of good and evil, where landscapes become simple reflections of human fears or desires. We feed throughout the story with these oppositions such as those between light and dark, up in the trees and down in the caves/dungeons, the story moves us basically in way where our eye can determine fear and terror, beauty and glory.
I feel sometimes that we have a kind of map of the unconscious. The duality happens when you take a good look at the description of Bilbo's house (almost heavenly clean and organized) and when you fall into the stincky caves of some ugly creatures. Then again we are in the magical forest/castle of the elves and then back again in the wicked spider trees.
I really don't think this is a good and evil battle. It's a "journey" that you have to win against yourself, find those bad chambers of hell and lock them for your own safety.
