What specifically is it about the old man that troubles narrator in The Tell-Tale Heart?

In Edgar Allan Poe's short story "The Tell-Tale Heart," the narrator is not specifically troubled by any particular aspect of the old man's appearance. Rather, it is the old man's one, pale blue eye that is the focal point of the narrator's obsessive hatred and fear. The narrator describes the eye as resembling that of a vulture—a carrion-eater—and as being an "Evil Eye." It is this eye, more than anything else, that drives the narrator to murder the old man and ultimately confess to the crime.