In Poe's celebrated short story "The Tell-Tale Heart," there is an abundance of evidence to suggest that the narrator is unreliable. From the start of the story, the narrator's sanity is called into question when he attempts to convince the reader that he is sane. A mentally stable person would not feel the need to justify or prove his sanity, and the narrator's admission that he has supernatural hearing is quite disturbing. Also, the narrator's syntax is fragmented and staccato, which is indicative of a mentally unstable, neurotic individual and undermines the narrator's argument that he is not mad.

As the story progresses, the narrator continues to provide clear evidence that he is unreliable and deranged. The narrator's reason for killing the defenseless old man is unusual and once again highlights his mental instability. A reliable, rational individual would not claim to love a person and proceed to murder them because of their "Evil Eye." Once the narrator commits the horrible crime, he proceeds to dismember the old man's body and hides his remains underneath the floorboards. The way in which the narrator casually discusses his disturbing crime and proudly boasts about hiding the evidence highlights his madness.

Once the police arrive, the narrator claims to hear the dead man's heart beating from underneath the floorboards, believes that the officers are mocking him, and completely loses his composure. The reader recognizes that the narrator is hallucinating and guilt-ridden, while the narrator seems to genuinely believe he has supernatural hearing. Overall, the narrator's rationale for killing the old man, his argument that he is sane, his cavalier attitude regarding his horrific crime, and his admission that he has supernatural hearing is clear evidence that the narrator is unreliable and mentally unstable.

In common with most of Poe's narrators, the narrator of "The Tell-Tale Heart" is indeed thoroughly unreliable. Why? Well, for one thing, he tells us that he's mad. Paradoxically, then, if he's telling the truth, then we can't fully believe him due to his fraught psychological condition. But if he isn't telling the truth, and he's perfectly sane, then we still can't believe him because he's just told us a flat-out lie about his being mad. Either way, then, he's as untrustworthy as they come.

Furthermore, the motivations given to us by the narrator to justify his wicked act—if indeed it took place—are pretty far-fetched, to say the least. He seriously expects us to believe that he brutally murdered this kindly old man, this man who'd never shown him anything but the greatest kindness, because he had an "evil eye." The very idea is just too ridiculous for words.

And then, to top it off, we're further informed that the narrator's been prompted to tell his story by the insistent sound of the old man's heart beating beneath the floorboards, where he buried his body in little pieces. Once again, if the narrator's really mad, then we can't trust him. But if he's telling the truth, his story sounds so utterly implausible that we must still remain on our guard.

The narrator of "The Tell-Tale Heart" is unreliable. He is trying from the very beginning of the story to make a case for his sanity, but the very story he tells completely undermines and is at odds with his assertions of sanity. Throughout his narrative, he recalls the events that led him to murder the old man and then confess. He attempts to illustrate his clearheaded reasonableness with examples. For instance, he tells us that he "loved the old man" and that the old man had never wronged him. This admission makes his murder of the man less, rather than more, understandable or sane. Why would you murder someone you loved who had done you know wrong—and merely over his eye, as the narrator insists is the case? 

He also asserts that he cannot be mad for the following reason:

Now this is the point. 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!

Yet, we know that madmen can be quite knowledgeable—and can proceed with caution and foresight. The narrator completely misses the point while thinking he is making a valid point: it is not how he proceeded but what he did—murdering an innocent man for no good reason—that attests to his lack of mental stability.

Finally, he hears the dead man's heart beating at the end, probably a manifestation of his own guilt. Yet, rather than assume he might be hearing things or hallucinating, the narrator violently insists that the police are mocking him:

They heard! —they suspected! —they knew! —they were making a mockery of my horror!—this I thought, and this I think.

The disjunction between what happens and how the narrator interprets it leads us to conclude he is unreliable. 

The narrator is a classic example of an unreliable narrator, because he is telling the entire story himself and there is no objective narration to back up his assertions. The first lines of the story show this, as the narrator is trying to explain that he is not mad, that he is perfectly sane, only that he is in the throes of some unnmamed illness that heightens his senses:

...why will you say that I am mad? The disease had sharpened my senses --not destroyed --not dulled them... How, then, am I mad? Hearken! and observe how healthily --how calmly I can tell you the whole story.
(Poe, "The Tell-Tale Heart," xroads.virginia.edu)

A reliable narrator would not be pressed to justify his act, but only to tell it simply and without embellishment. Since this narrator cannot explain his actions without constantly explaining himself, he can't be fully trusted; furthermore, his later mental breakdown as he imagines the beating of the old man's heart shows that his mind is not healthy. If the story is taken allegorically, it is possible that the crime was imaginary and the narrator explaining his motivations in a sanitarium. In this story, the unreliable narrator is the key to both the crime itself and its solution; had the narrator had less guilt, and more mental stability, he might have gotten away with the murder much as Montresor did in Poe's "The Cask of Amontillado."