The narrator believed that he could hear the old man's heart beating in "The Tell-Tale Heart." Why?
First it is the evil eye; then it is the heart beating. It is obvious that something is amiss in the mind of the narrator in the chilling horror short story by Edgar Allan Poe, "The Tell-Tale Heart." The mental instability of the narrator makes its appearance from the first lines of the story, and continues through his obsessive compulsion to rid himself of the evil eye.
TRUE! nervous, very, very dreadfully nervous I had been and am; but why WILL you say that I am mad? The disease had sharpened my senses, not destroyed, not dulled them. Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell.
The narrator admits to having suffered from some disease which has left him hearing things not of this world. His obsession with the old man's eye proves that his vision has also been affected. But it is his mind that is most in need of help. It becomes even more obvious when he soothes his own anguish by killing and dismembering the old man.
Later, when he hears the "beating of his hideous heart," it is the guilt felt by what is left of his conscience that causes him to hallucinate in this manner. The man is insane, and his act of murder has not placated his abnormal desires.