The house of Usher, both in terms of the Usher family and the house it self were confused by the passersby as "one and the same". This was because, as you read, the story shows that the family had a history of disease, death, entrapment, insanity. This is a compilation of the scariest demons that humans have to deal with from time to time. In Usher's case, Roderick and his family were all representatives of such demons, of such imperfections. The house decayed as Roderik decayed, and both the house and the owner were going downhill. Finally, when the narrator escapes, the house implodes on its own. Similarly with Roderick and humans dealing with their inner torments, they are also bound to implode and destroy themselves. (And those around them can help, just as much as they don't fall right with them)
From the time that he receives the desperate letter of Roderick Usher, the narrator has a sense of foreboding about what Roderick describes as "acute bodily illness" and a "mental disorder" that oppresses him. These fears begin the journey into the mind of Roderick Usher.
When he arrives at the decaying mansion of Usher, the narrator is amazed at the change in his boyhood friend. There are inconsistencies in his behavior, as his moods vary from lively to sullen; then, too, he is alternately decisive and indecisive. There are wild swings in his mood and in his tone when he speaks,
His voice varied rapidly from a tremulous indecision...to that species of energetic concision.
Roderick Usher also holds rather superstitious ideas about the mansion itself, ideas which exert a strong affect upon him since he has not left his home for many years. Also, the grayness of the sky and the dim lake depress him. Indeed, there exists for Roderick Usher a pathetic fallacy of nature with his house and his nervous condition that grows more and more melancholic. For, Usher believes that the peculiar condition of his house with its fissures in the wall and its decay have taken over and deteriorated his own mental and moral condition.
Additionally, Roderick expresses himself in his art and music, and such acts of expression do soothe Roderick. Nevertheless, Usher suffers from a nervous disorder which intensifies his ability to hear, and his sight is so sensitive that his eyes are bothered even by dim light. With such heightened sensations, Usher tells his old friend,
"I shall perish," said he, "I must perish in this deplorable folly....I fear that the period will...arrive when I must abandon life and reason together, in some struggle with the grim phantom, FEAR."
As Roderick struggles with this fear, he tells the narrator that if his twin sister Madeline were to die, he would be the last of the Ushers because he has no cousins. Madeline's strange illness is, thus, extremely disturbing to Roderick. Later, Usher writes a poem which indicates his disturbed state as he is tormented by the conditions of the mansion and the "cataleptical character" of his sister. The finale of his poem and its invasion of "evil things" suggests, too, the increasingly fragile psyche of Roderick Usher that attempts to hide those "things" that affect his sanity.
The climactic moment in which "the grim phantom FEAR" arrests Roderick comes when Madeline rises like a phantom from the dead and holds her brother in a death embrace. While their souls are reunited, Roderick loses all hold upon his "life and reason," just as he has predicted, and the tortured journey of his mind is ended.