In "The Tell-Tale Heart," Poe narrates the story through the first person point of view, a narrator who comes across as being completely unreliable and even mad. The narrator's roommate, an old man, has a filmy eye. It disturbs the narrator to the extent that he begins planning murder. The filmy eye is the reason the narrator gives for why the old man must die, but the discerning reader would have already seen this for what it really is--the narrator is insane, really crazy.
In fact, the narrator even addresses this:
"You fancy me mad. Madmen know nothing. But you should have seen me. You should have seen how wisely I proceeded..."
The inherent creepiness of this is that the narrator's tone at the beginning of the story is so matter-of-fact, as he is spying on the old man, creeping into his room at night to look at him while he sleeps, only to ask about his health and well-being the next day at breakfast.
The narrator wants to prove his sanity to the reader, so he goes about this by trying to convince the reader of how methodically he throught through his plans of premeditated murder.