In this scene, The narrator thinks the old man's eye is a vulture and wants to get ride of it. To do this the narrator is stealthily watching the old man sleep waiting for the perfect moment to kill the poor old man. Then the narrator finds the perfect moment and kills the old man.
The Police arrive and suspect foul play the the narrator is confident that he won't get caught because he hid the body. The narrator says in confidence that he's watching out the house for the old man because he's out of the country.
In this scene, the narrator's still in confidence that he won't get caught, but then he begins to feel guilty to a point where he begins to hear a sound that wouldn't stop. This sound made the narrator go crazy so much that he thought the police offices were mocking him, so he confessed to the crime he committed.