Madness and paranoia are the dominant impressions I get from "The Tell Tale Heart." Just look at the opening line of the story, "True! nervous, very, very dreadfully nervous I had been and am, but why WILL you say that I am mad?" It is clear that our narrator is neither stable nor reliable.
He then explains to the reader that he is not mad, for listen to how calmly and rationally he is going to relate the tale. This is ironic, for the more we read, the more we realize that the narrator is indeed mad. Not how stealthy he opens the old man's bedroom door each night to look in on him and his "evil eye." Clearly, these are the actions of a madman.
After killing him, chopping him up, and concealing the body beneath the floor boards, the narrator believes he has committed the perfect crime. But it is his paranoia (or guilt) that begins to get the best of him.
He seems to live up to his claim of sanity, bidding the police to entire his house and search it thoroughly. He is almost daring them to find a clue. At this point the reader can see how paranoid he has become in convincing the police of his innocence, yet it is this that is his undoing. As his paranoia (or guilt) is personified by the beating of the dead-man's heart, causing the narrator to confess in front of the police. The conclusion of the tale, like the opening, leaves the reader with the impression of insanity and paranoia.