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.
Whenever you are asked this kind of question about a text the main thing to think about is what the concerns are of the text you are studying. However, it is slightly misleading to think that any text is built around one single theme. The vast majority of texts have a multiplicity of themes. Considering "The Fall of the House of Usher," therefore, I will talk about one of the themes which is madness, however you might want to analyses other themes such as the nature of evil as presented in this excellent tale.
When we think about insanity in this story it is clear that this theme emerges from the ever-more unstable condition of Roderick Usher who is suffering from an unspecified nervous condition which is very mysterious. The narrator describes him as "a bounden slave" and he is told that his condition is related strongly to the actual House of Usher that has already been described to us looking so grey and grim. Yet there is also a link between Roderick's illness and his sister's strange malady, that causes her to become cataleptic. Roderick, desperately, tries to use the narrator's company to engage in other activities to ease his malady, yet in spite of these efforts, his actions become more and more hysterical, and we as readers, just like the narrator, are left to wonder about what is behind these increasingly frenzied actions. It is a mark of Poe's excellence that the reason for this insanity always remains ambiguous (possible incestuous relationship or evil, vampiric possession?), yet we would be wrong to focus so strongly on the theme of evil in this story that we forget the questions it raises about the frenzied descent into the maelstrom of madness that we are presented with by Roderick Usher.
Hope this helps - you might want to re-read the story and think about what it says about evil now. Good luck!
While there exist differing interpretations of Edgar Allan Poe's "The Fall of the House of Usher," certain aspects are undeniable. One such aspect is that of the characteristic Dark Romanticism of the author. Certainly, the forces of the natural world work against, or at least, mirror the forces working within the characters of Poe's narrative. Thus, the Gothic aspect of the story prevails throughout the setting as well as within the character development.
With the double entendre of the title as a reinforcement of this parallelism, there is an effective suggestion of evil and madness, two themes which are evinced in Poe's story. For instance, as the narrator approaches the house of the Usher family, he becomes aware of the atmosphere of dread and menace of the "insufferable gloom [that] pervaded [his] spirit." Such details as the "decayed trees with an utter depression of soul," and the "iciness, a sickening sickness of the heart" that the narrator experiences parallel the decadence of the figurative family tree of Roderick and Madeline Usher, as well as the degeneration of the Usher family, which leads to the bizarre illnesses of the siblings.
Furthermore, as the mind and the body of Roderick and Madeline respectively deteriorate, so, too, does the mansion decay. A fissure in the wall is observed, fungi develops on the stones of the exterior. In Roderick his acute sensitivity to noises disturbs him excessively. This "sentience of all vegetable things" mirrors this sensitivity of Roderick. Likewise, Madeline's corpse in the coffin mirrors the face of her twin brother who possesses a "restrained hysteria in his whole demeanor." As Roderick reads to his friend later on, Roderick starts, "Madman! I tell you that she now stands without the door!" Thus, the madness and evil of the atmosphere and the happenings of the interior of the mansion, meet forces and join together for the horrifying end as Madeline returns from the coffin to take in her brother with an eerie and deadly embrace. Simultaneously, the fissure widens and the "mightly walls rush asunder," destroying all of the House of Usher; the double entendre of the two meanings for the "The House of Usher"--the mansion and the family--and the parallelism of evil with madness is complete.