As the story opens, Poe's unnamed narrator has decided to get rid of an older man with whom he lives; whether it is his employer, his uncle, his grandfather, we do not know. There is no reason for this decision, except for the fact that the narrator dislike the way the old man looks at him with his "vulture eyes.".
"True! - nervous - very, very dreadfully nervous I had been and am; but why will you say that I am mad?".
The Tell-tale heart is all about how the narrator is trying to convice us that he is not mad, but he only proves that he clearly is.
The narrator has a deep sympathy for the old man, even though he is scheming to kill him. "I loved the old man. He had never wronged me. He had never given me insult. For his gold I had no desire" It seems that he is motiveless, and unconnected with passion or profit, but in a deeper sense, the murder does have a purpose: to ensure that the narrator does not have to endure the haunting of the Evil Eye any longer. The truth in the matter is that he knows he cannot bring himself to admit to the point that only a mad man would kill someone just because of the way their eyes looked. " It was not the old man who vexed me, but his evil eye." To the madman, this is as good of a reason as any; in the mind of a mad man, reason does not always win out over emotion. .
He tries to prove that he is not mad by painting a vivid and remarkable picture of the story and by telling how he used logic when he planned the homicide. He believes that since he is able to recollect and present every detail of the events that took place proves that he is not insane. He believes that he is sane because of the manner in which he carried out the crime of murder. The madman is quite logical and reasonable, that is true, but in an contradictory way; the narrator has the facilities to reason and plan but not to distinguish right from wrong.
Another thing that shows he is mad is that he feels no shame for his deed; even when the police come, he is the picture of calm.