I love The Master and Margarita by Bulgakov, and the character of Woland, who is basically the Devil though never explicitly referred to so. I always liked this speech of his about the place of evil in the grand scheme of things, and the way it’s inseparable from good:
You spoke your words as though you denied the very existence of the shadows or of evil. Think, now: where would your good be if there were no evil and what would the world look like without shadow? Shadows are thrown by people and things. There’s the shadow of my sword, for instance. But shadows are also cast by trees and living things. Do you want to strip the whole globe by removing every tree and every creature to satisfy your fantasy of a bare world?