Poe provides many context clues through out "The Tell-Tale Heart" to suggest, that despite the narrator's assertions of sanity, that he is in fact insane. At the very beginning of the tale, the narrator insists:
TRUE!—NERVOUS—VERY, very dreadfully nervous I had been and am; but why will you say that I am mad?
This introduction alone strikes the reader as being a little crazy, because an average, regular person would not feel the need to verify their own sanity, unless it was possible that he might have suffered from mental illness in the past. The narrator's voice feels as if he is trying too hard to convince the reader of his mental stability, especially as he describes how careful he was to be nice to the old man the week before he carried out his plot to murder him:
You fancy me mad. Madmen know nothing. But you should have seen me. You should have seen how wisely I proceeded—with what caution—with what foresight—with what dissimulation I went to work! I was never kinder to the old man than during the whole week before I killed him.
As he proudly describes what care he took to proceed with his murderous plot, the narrator come across as extremely unreliable, filling the reader with doubts of not only his veracity but also his mental stability. Moreover, as the story unfolds, the narrator's actions further suggest his insanity; for example: 1) sitting down on the mattress after he smothers the old man; 2) hiding the body inside the house; 3) his imagined hearing of the still beating heart. Poe's cleverly constructed story clearly reveals the mind of a completely psychotic killer.