Evil is for the most part the result of a superabundance of good, though it quickly gains relative autonomy and forgets its origins.   Good is re-established as soon as an evil entity is designated as an obstacle, so that it can become an instrument for discipline.  This is why evil is associated with darkness: the only evil in the world is evil that is ~unseen~.  The horrifying thing is that only a tiny portion of this ~can~ be seen and designated in this way, so that unseen evil effectively rules the world