Simply put, the old man was killed by the narrator. The first line "True!—nervous—very, very nervous I had been and am; but why will you say that I am mad?" alerts the reader that the speaker is clearly unreliable, and we are alerted to the fact that something unpleasant has occurred. Any act that would label someone as "mad" for having committed can't be good news. The narrator goes on to tell us that the murder was carefully planned beforehand, & it seems to have been sparked by one trivial physical trait: the cataract over the old man's eye. For an unknown reason, the old man’s cloudy, pale blue eye has incited madness in the narrator. He believes that only by killing the old man can he get rid of the eye’s overpowering malignant force.
Thus, he plans out his crime. For a whole week, he enters the old man's room at night, but the victim remains sleeping each time. The narrator cannot bring himself to kill the man without seeing that which drives him into madness, so he continues entering the room until the old man wakes. On the eighth night, he wakes up and cries ‘‘Who’s there?’’ The narrator does not respond, but instead waits silently for an hour. Eventually, the narrator shines his lamp on the old man’s eye. Heis overcome with madness at the ‘‘damned spot,’’ but he soon hears the beating of a heart so loud that he fears the neighbors will hear it. With a yell, he leaps into the room and kills the old man.