The narrator said that he is sick but not crazy, just nervous. He says the illness helped him not turned him insane.
The narrator loved the old man but hated his eye. He compared it to a vultures eye. He said he felt nervous around it.
So the narrator decided to kill the old man. He would go to his room every night and open a lamp very little so that it fell on the old mans eye. But it never did so he kept going to see the old man sleep.