In “The Tell-Tale Heart,” the narrator, who claims to be sane but whose actions demonstrate otherwise, decides to kill the old man he takes care of.
The narrator says he does not have any ill feelings toward the old man, nor does he have any interest in his money. He decides to kill the old man simply because he is unnerved by his vulture-like eye.
The narrator proudly tells us how carefully and methodically he planned the old man’s murder. During the week prior to the killing, the narrator is exceptionally kind and loving toward the old man. He says, “I was never kinder to the old man than during the whole week before I killed him.”
The narrator, who is clearly insane and suffers from an unspecified disease, is unreliable, which makes his behavior and motivation difficult to discern. There is a lack of alignment between his words and his actions. For example, he claims to love the old man but devises and carries out a plan to kill him. Similarly, the narrator frequently tells us he is not insane, while engaging in obviously insane behavior.
It cannot be known for sure why the narrator is extra kind to the old man in the week preceding the murder, but it is likely that he behaves this way to avoid arousing the old man’s suspicion. Perhaps he wants to kill the old man quickly and suddenly and spare him the foreknowledge of his death and the fear that would accompany it. Perhaps he does not want the old man to have a chance to fight back or thwart his insidious agenda. Another possible reason for the narrator’s kindness is guilt. Perhaps, on some level, he feels guilty for planning to kill someone who has been kind and good to him.
Poe's intentional ambiguity leaves us to draw our own conclusions about the narrator's behavior.