Madness and the dark side of human nature certainly seem to be two of the major themes of Poe's classic short story. I've included some examples below.
- The murderer tries to explain to the reader that his careful plans are not a sign of madness, but his actions suggest otherwise.
- The narrator claims to hear things that emit from unearthly sources.
- The narrator claims that he has no specific reason to kill the old man other than his "vulture eye."
- His careful preparation to kill the man in his sleep shows a dark side of the man's character.
- The dismemberment of the body is a particularly gruesome task that could only have been carried out by a man filled with either mental instability or pure evil.
- His belief that he can still hear the heart beating is further proof of madness.