The narrator has been sneaking into the old man's room for a week, each night at midnight, hoping to be able to kill the old man, but the narrator has not been able to do it yet because the old man has been asleep each night. The narrator must see the old man's "vulture" eye in order to get upset enough to commit the murder.
On this, the eighth night, the narrator feels certain that he will be successful. He believes that the old man is lying in his bed, completely oblivious to the fact that the narrator is creepily listening outside his bedroom door. As is his habit, he has opened the old man's door just a crack, and he hears the old man shift his body in the bed. However, the narrator does not feel any fear or concern of being discovered because it is so very dark. He knows the old man cannot see him. The pitch darkness does seem to be incredibly foreboding, apparently foreshadowing the old man's death. We also see just how terribly self-centered, even narcissistic the narrator is, as he focuses on his feelings, the fact that he is not afraid—why would he be afraid in this moment? It is the old man who would feel fear.