How is the narrator really crazy in "The Tell-Tale Heart"?
Another clue we get about the narrator's unstable mentality in this story is that his reason for killing the old man is that he does not like the old man's eye. When the narrator ponders his reason for committing murder, he says,
There was no reason for what I did [....]. I think it was his eye [....]. When the old man looked at me with his vulture eye a cold feeling went up and down my back; even my blood became cold. And so, I finally decided I had to kill the old man and close that eye forever!
Now, there is probably never a great reason to kill another person, but some reasons are certainly more understandable, or more "sane," than others: self-defense, a person had wronged you terribly, a person poses a major danger to you or a loved one, and so forth. However, the narrator says that he literally has no reason to kill the old man other than that he hates the man's eye. This is a pretty unacceptable reason to kill a person, at least for a sane person.
Further, when he describes the way he prepared to commit the murder, the narrator gives us more clues about his mental health. He says that he looked in on the old man at midnight every night for a week, but on the eighth night, he says, "I was more than usually careful as I opened the door. The hands of a clock move more quickly than did my hand." First of all, this is super creepy, but it is also very odd. Either he actually moves so slowly that the clock's hands move more quickly than his do, or his perception of reality is so warped that he only thinks this is true when it is not. Either way, he is clearly not in his right mind.
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.