In Poe's "The Fall of the House of Usher," perhaps what the reader sees is the deterioration of Roderick and Madeline Usher's family, specifically its health: physical and mental. The cause of these goes back to tradition and conformity to tradition.
It would seem that the brother and sister may have had an incestuous relationship. Incest, among its social taboos, also causes birth defects, disease, and/or madness. If their parents had also practiced incest, this would have easily caused problems for their children. Madness (or evil) seems to be the central focus of the story as Roderick's behavior becomes more unbalanced and frightening as the story progresses, and even Madeline's health deteriorates.
In terms of tradition, especially in long-gone traditional houses of power, marriages of cousins would take place to keep the control within one family. This, however, often caused a high mortality rate among the offspring of these "couples," as well as madness.
In Madeline's case, she becomes so ill that her brother mistakenly buries her alive believing she is dead. When she reappears, having escaped from her entombment, madness swirls around the pair, the weather seems to have gone wild, and the unnamed narrator flees, only to soon witness the house's destruction when lightning bolts split the structure in two. The family name of the last two heirs of the Usher line is destroyed at the end as well.
Poe speaks to the nature and causes of evil. What has gone on in this house might have been considered by Poe to be evil--whether for the sake of evil itself or because of unnatural behaviors, we cannot be sure.