Human Nature as Evil

Suppose it is up to you as to where the "evil" lurks in this story. The narrator plays the straight or neutral character. Any ghostly funny business belongs to Usher and the house he lives in. We never know what exactly Usher did with his sister; incest is a definite subtext here. Whatever their relationship, she does end up a zombie and the destruction that is evil in nature, pretty much everything in the estate including Usher and his sister, rots away into destruction.