The old mans eye is torturing the narrator. He cant stand the eye piercing into his soul much longer.
Again and Again
The narrator finally snapped. He killed the old man in hopes to get rid of the horrid eye.
The police showed up at his house responding to a complaint about a scream. The narrator let them inside feeling confident that he hid the body and evidence well. When the officers left the narrator felt as if they knew what he had done and were mocking him outside the door. The narrator then confessed to the crime.
The narrator repeats the sound of the old mans heart beat. This makes the narrator sound unreliable because you can not hear a humans heart beat with just using you ear.
The narrators anxiety has taken place as the old mans heart beat. Even when the old man was dead the heart beat still continued to beat. This was the narrators guilt in committing the crime.
Being mentally unstable will make you do some crazy things.