The actions portrayed by the narrator in Edgar Allan Poe's "The Tell-Tale Heart" seem to prove two of the main themes of the story--those of guilt and insanity. Although the narrator tries to convince the reader throughout that he is quite sane, his actions tell a different story. From the very first lines, we know that there is something wrong with him.
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. How, then, am I mad?
Certainly not the words of a sane man, the narrator's later discussion about the man he would later murder--his love of the old man and his "vulture-eye"--further prove that his motivations are foundless.
The theme of guilt is addressed primarily by the resulting beating heart that the narrator hears long after the old man has been killed. To the madman, it is an audible sound, though it cannot be heard by the policeman in the same room. The narrator's guilt also reflects the "doppelganger" effect, in which the old man functions as the narrator's double or foil. Assuming the old man is the narrator's doppelganger,
"... he kills the old man because he cannot stand himself, perhaps fearing becoming old or disfigured like him."