The doctrine of the Fall (both of man and of some ‘gods,’ ‘eldils’ or ‘angels’) is the only satisfactory explanation. Evil begins, in a universe where all was good, from free will, which was permitted because it makes possible the greatest good of all. The corruption of the first sinner consists not in choosing some evil thing (there are no evil things for him to choose) but in preferring a lesser good (himself) before a greater (God). The Fall is, in fact, Pride. The possibility of this wrong preference is inherent in the v. (very) fact of having, or being, a self at all. But though freedom is real it is not infinite. Every choice reduces a little one’s freedom to choose the next time. There therefore comes a time when the creature is fully built, irrevocably attached either to God or to itself. This irrevocableness is what we call Heaven or Hell. Every conscious agent is finally committed in the long run: i.e., it rises above freedom into willed, but henceforth unalterable, union with God, or else sinks below freedom into the black fire of self-imprisonment. That is why the universe (as even the physicists now admit) has a real history, a fifth act with a finale in which the good characters ‘live happily ever after’ and the bad ones are cast out. At least that is how I see it.
The Collected Letters of C.S. Lewis, Volume II