The narrator planned to commit the perfect crime and failed. "The Tell-Tale Heart" belongs to that sub-genre of stories called "Perfect Crime Stories." Poe may have actually invented this sub-genre, as he invented so many others. In the "Perfect Crime Story" the perpetrator thinks he is so clever that he can get away with murder, but something always gives him away. In Poe's story the murderer gives himself away because his guilt, fear, and mental instability cause his imagination to run away with him. He thinks he hears the victim's heart beating, and the sound begins to seem so loud to him that he believes the investigators must be hearing it too but are only playing mind-games with him, waiting for him to break down and confess. The moral of Poe's story, as with all "Perfect Crime Stories," is that there is no such thing as a perfect crime. Poe wrote one famous story in which the murderer planned a perfect crime and actually got away with it. That story is "The Cask of Amontillado."