How is Roderick Usher's "mental illness" schizophrenia? (or is it not?)
The fact that Roderick "hears things" puts him in the category of a schizophrenic- that is, until the reader learns at the very end of the tale that the sounds were indeed coming from his sister Madeleine, who was buried alive and trying to escape. Finally, she does so, but the fright of the very sight of her is the undoing of Roderick, and the house (already cracked in several places) crashes to the ground simultaneously.
This is a very hyperbolic dramatic ending for a sordid tale suggesting many things "unnatural", including the relationship between the brother and the sister. Other critics interpret the house as a malign force exercing its influence over an otherwise normal relationship.
There are other signs, though, of mental disturbance in the character of Roderick, particularly his mood swings. This would suggest a bipolar personality disorder, not quite as bad as schizophrenia but a serious disorder just the same.
Note that the Ushers' house serves as a symbol of something unhealthy within the household itself which progressively manifests itself - first, a distinct line first visible from the outside, running vertically from the ground; then, a crack ever widening, with the walls even heaving under the strain; finally, the house itself quaking, then literally toppling over into ruin.