The narrator in the story explains in vivid detail of the old man's eye. "He had the eye of a vulture--a pale blue eye, with a film over it."(Poe 80) And because of the eye, it made the narrator go crazy.
When the narrator was talking to the police he started to get very nervous "I talked more quickly --more vehemently; but the noise steadily increased." (Poe 81) And because of the heart beat he heard, he felt guilt
Every night for seven night the narrator kept staring and staring at the old man's eye." And I did this for seven long nights," (Poe 78) And because of the eye that vexed him, he kept staring at that eye before he killed him.