After putting Madeline in the vault, Usher’s madness begins to increase. Why does he get so much worse?

Narrator gets his first good look at the face of the deceased. He is struck by how similar in appearance she and Roderick are. He learns that they were twins and that there had always existed some kind of intangible bond between them. In the days that follow, the Narrator notes the increasing madness of Usher: his skin grows whiter, his ordinary occupations are forgotten, and he roams through the house or stares into space for hours and hours. What frightens the Narrator even more is that he too is beginning to feel "infected" by Usher's condition. The Narrator fears that he too may be going mad. Madeline's questionable relationship with Roderick represents the decay of Roderick's world. The unwholesome and the diseased are beginning to crumble.