What does the story "The Fall of the House of Usher" by Edgar Allan Poe examine?(Please include quotes in your explication)

The story, "The Fall of the House of Usher" by Edgar Allan Poe,
deals with two themes: evil and madness.

It seems as if Roderick
Usher is going insane. He is weighed down by worry, depression and hysteria. However, I have felt
when reading this that the undercurrents in the story tie the themes together, and that the evil
practiced in the house is responsible for Roderick's ensuing madness, among other things.
(Learning he has buried his sister alive also feeds Roderick's disconnect with a healthy mental
state.)

It has been suggested that the close ties that exist between
Roderick and his sister Madeline are based on an incestuous relationship. This was not an
uncommon practice among "great" families of England, especially the royal houses, in order to
keep power within that family.

If this is the evil that exists, it
explains the insanity as well, as incestuous relationships can cause physical and mental defects
in the offspring of such a joining, and as the House of Usher is dissolving away, one has the
sense that this brother and sister may be the result of a long line of ancestors who also were
involved in this practice.

Poe, then, may be speaking to the issue
of incest within families, as well as the issue of mental illness. (The mentally ill were not
well-cared for at that time, as little was understood about the
brain.)

Poe himself dealt with serious depression, and many of the
characters he writes about (e.g., "The Tell-Tale Heart" and "The Cask of Amontillado") are
insane.