The narrator is a classic example of an unreliable narrator, because he is telling the entire story himself and there is no objective narration to back up his assertions. The first lines of the story show this, as the narrator is trying to explain that he is not mad, that he is perfectly sane, only that he is in the throes of some unnmamed illness that heightens his senses:
...why will you say that I am mad? The disease had sharpened my senses --not destroyed --not dulled them... How, then, am I mad? Hearken! and observe how healthily --how calmly I can tell you the whole story.
(Poe, "The Tell-Tale Heart," xroads.virginia.edu)
A reliable narrator would not be pressed to justify his act, but only to tell it simply and without embellishment. Since this narrator cannot explain his actions without constantly explaining himself, he can't be fully trusted; furthermore, his later mental breakdown as he imagines the beating of the old man's heart shows that his mind is not healthy. If the story is taken allegorically, it is possible that the crime was imaginary and the narrator explaining his motivations in a sanitarium. In this story, the unreliable narrator is the key to both the crime itself and its solution; had the narrator had less guilt, and more mental stability, he might have gotten away with the murder much as Montresor did in Poe's "The Cask of Amontillado."
What details heighten the mood of horror in this passage?
the narrator was mentally disorder. he was a physco person. so one should not belive on his evidences he is unreliable.