The correct answer is that - murder causes an all-encompassing guilt. The theme of the whole story is how a human being cannot possibly bear to hide such a terrible secret and live with so much guilt. The narrator of the story killed a man and buried him under the floor, but he constantly kept hearing the dead man's heartbeats, which is, of course, impossible, but as he was feeling so guilty for having murdered a person, his mind led him to insanity.