What does usher believe will eventually kill him?

Usher believes that his death will ultimately come from a combination of his own mental and emotional turmoil, particularly stemming from his family's dark history and the pervasive fear of hereditary illness. He feels trapped by the weight of his ancestry and the decay surrounding him, leading him to a sense of inevitable doom. This belief is intertwined with his deteriorating mental state, exacerbated by the oppressive atmosphere of his decaying home.