The narrator’s tone is arrogant and excited. He is excitable because he is mentally unstable. Yet he is arrogant enough to believe he is right.
The narrator in “The Tell-tale Heart” is an unreliable narrator. We realize from his tone that he is mad. The narrator is obsessive and paranoid. He describes the killing of his roommate and the consequences in a matter-of-fact way, but in a very excited way.
The narrator goes to great lengths to prove that he is not mad, but almost everything he says confirms that he is not in his right mind. He tells us that he had nothing at all against the old man.
It is impossible to say how first the idea entered my brain; but once conceived, it haunted me day and night. Object there was none. Passion there was none. (enotes etext p. 4)
Yet the old man has to die! The narrator explains that it is because he has an evil eye. We can’t let that stand!
Oh, you would have laughed to see how cunningly I thrust it in! I moved it slowly—very, very slowly, so that I might not disturb the old man's sleep. It took me an hour to place my whole head within the opening so far that I could see him as he lay upon his bed. (p. 4)
The narrator painstakingly describes how it took him an hour to stick his head in the door. He is thoroughly excited. An example of this tone is the use of exclamation points and words like “Ha!” with short, choppy sentences.
The tone of the story is a choice made by Poe to ensure that his narrator seems mad, but nearly lucid. The narrator is obsessed and excitable, but there is enough normalcy there that it makes the ending more believable. We realize that this man could hold a conversation with the police without arousing suspicion for only a short time, and he would not realize it when they began to suspect him because he is so arrogant.
The tone also adds to the suspense, because we know that this arrogant and excitable man is up to something. Early on we are told that he killed the man, and the tone makes us interested in learning how.
The narrator's tone in this story is quite overwrought, leading to a climax at the end when he shrieks out his confession to murder. He is disturbed in his mind, and this is conveyed in his nervous agitated manner. He often gives way to exclamations, although he also tries to speak quite rationally and logically. He also addresses the reader directly on several occasions, creating the sense of a more intimate and close audience. This psychologically intense tone conveyed through first-person narrative is common to several of Poe's short stories like 'The Black Cat' and 'Berenice', which also feature acts of murder and mutilation by obviously psychologically-disturbed individuals.