When considering symbolism in Gothic Literature (especially Edgar Allan Poe) I think it is safe to say the following ideas are probably represented somewhere:
- darkness (physical, mental, emotional, moral)
Consider Roderick's characteristics:
- hypochondriac and disorder of heightened senses
- keeps to himself
- generally kind but clearly hiding something
I think it would be safe to argue that he is symbolic of a family secret, an evil. Most probably interpret it as incest. He is the last of the Usher line, which means the secret dies with him.