Like many of Edgar Allan Poe's stories, The Tell-Tale Heart contains more than one theme, including his mortal fear of being buried alive. The most prevalent theme, however, is Guilt.
The nameless narrator speaks of an undefined loathing for the Old Man who lives in his house, and of the Old Man's staring eye, which watches him incessantly. Eventually, the narrator kills the old man in the night and dismembers the body, hiding the corpse beneath the floor. When Police come to investigate a neighbor's report of a scream, the narrator entertains them on the spot, but grows more and more nervous as he imagines the old man's heart still beating below them. Eventually, he works himself into a frenzy and tears up the floorboards himself.
The narrator speaks constantly of his sane state of mind, and yet he himself reveals his perfect crime. In his guilt over killing a man who, he admits, had never wronged him, he mistakes every sound and sight for proof that the police already know and are just toying with him. It is likely that the heartbeat he hears is his own, beating in his ears.
Near the climax, the narrator explains,
Anything was more tolerable than this derision! I could bear those hypocritical smiles no longer! I felt that I must scream or die!
And scream he does, exposing his crime. While he never denies the crime itself, he denies that the commission of it was in any way immoral. He has taken a life for no purpose but his own satisfaction, yet refuses to understand or accept his guilt even as it undermines his calm.