One of the first pieces of evidence that indicates that the unnamed narrator is insane is his obsession with the old man's "vulture" eye. The narrator explains his reasoning by saying,

Object there was none. Passion there was none. I loved the old man. He had never wronged me. He had never given me insult. For his gold I had no desire. I think it was his eye!

The narrator is clearly mentally unstable and even contradicts himself by stating that he "loved the old man." Why would anyone want to murder the person they claim to love? It is also concerning that the old man's pale blue eye is the primary reason the narrator is motivated to kill him.

In addition to the narrator's questionable motive, the narrator also comes across as desperate. The narrator is continually attempting to prove his sanity. Why would a rational, stable person need to convince someone that they are sane? Another piece of evidence that indicates the narrator is insane concerns the syntax of his narrative. The narrator speaks in fragmented sentences, which create a halting cadence that suggests agitation. This makes the narrator come across as neurotic and mentally unstable.

One of the most significant pieces of evidence that indicates the narrator's insanity is the brutality of his crime. The narrator not only stalks and suffocates the old man, but also dismembers his body and places his limbs underneath the floorboards of his home. Only a mentally insane individual would be capable of committing such an atrocious act. The narrator once again displays his insanity by claiming to hear the old man's heartbeat.

There are several red flags that suggest the narrator is insane and mentally deranged in the first paragraph of Edgar Allan Poe's "The Tell-Tale Heart." The abrupt, staccato sentences, repetitive questioning, and use of the words "nervous" and "mad" immediately give the reader the impression that a neurotic, mentally unstable individual is speaking. The fact that the narrator is attempting to prove his sanity is also unsettling, as well as his apparent supernatural hearing powers. Mentioning that he could hear "all things in the heaven" and "many things in hell" is clear evidence that the narrator is mentally unstable and insane. The narrator goes on to contradict himself by saying that he loved the old man before mentioning that he decided to kill him in the same paragraph.

The narrator's reason for killing the old man, which is the old man's "Evil Eye," is further evidence that he is insane. When the narrator enters the old man's room to murder him, he mentions that he understood the old man's terror because he had also experienced a "dreadful echo" in his "own bosom" at night. This statement is particularly disturbing and coincides with the other numerous pieces of evidence that suggest the narrator is insane. The narrator's frequent pleas for his sanity and excuse that he simply experiences "over-acuteness of the senses" also indicates that he is mad. The final piece of evidence that proves the narrator is mentally deranged is the fact that he believes he hears the deceased old man's heart beating beneath the floorboards.

One of our first clues that the narrator is not in his right mind is that he has become obsessed with the old man's "vulture eye," as he calls it.  He says that once he conceived of the idea to kill the old man, he becomes obsessed with it.  However, he says, "Object there was none.  Passion there was none.  I loved the old man.  He had never wronged me.  He had never given me insult.  For his gold I had no desire."  First, he wants to murder the old man, a big clue to his insanity on its own.  In addition, the only reason he has to kill him is to get rid of the man's eye; it freaks the narrator out.  If there is such a thing as a good reason to kill someone, this is not it.  We might be able to comprehend greed or passion or something like this, but murdering someone because their eye is offensive to you is not sane.

Another clue is the narrator's obsessive, seemingly compulsive repetition, night after night, of approaching the old man's door "just at midnight" and the slow process of inserting his head and lantern into the old man's room.  A totally sane person would simply not feel such satisfaction in this repetition, or the need to continually repeat the same process again and again.  Further, he cannot kill the old man when he is sleeping, with his eye closed.  It is the rage produced by the sight of the eye that finally enables the narrator to complete the deed.

Further, the narrator believes that he is hearing the old man's heartbeat, first across the room when the old man lies awake and listening, next after the narrator has actually killed and dismembered the man and buried him beneath the floorboards.  A sane person would realize that someone's heart cannot continue to beat after they have died.