I want to focus on the first paragraph of this story in which Edgar Allen Poe sets the tone. In this opening paragraph, the first-person narrator repeatedly uses words like "nervous" and "mad." In addition, the narrator's self-described mental state, including his repeated discussion of his hearing-related words, gives entire story a feeling of lunacy.
In the first paragraph, the narrator constantly describes things he hears. He discusses his "acute" sense of hearing and how his "disease had sharpened his senses." In addition, the narrator describes how he has "heard all things in the heaven and in the earth" and has heard "many things in hell." In addition, the narrator consistently describes himself as not "mad." He claims that he can tell the story of his murder "healthily."
All these word choices add to the idea that this man is, in fact, crazy. These words create a tone in which the reader knows he will dive into this man's mad mind.