In "The Tell-Tale Heart", the narrator is not a reliable source of information. He is clearly unstable and suffering from some form of mental illness, as evidenced by his obsession with the old man's eye and his erratic behavior. His paranoia and delusions make it difficult for the reader to trust his account of events. Additionally, his constant assurances that he is not mad only serve to further undermine his credibility. Ultimately, the narrator's unreliability adds to the suspense and tension of the story, as the reader is left unsure of what is real and what is a product of the narrator's disturbed mind.
