At the beginning of the story, the narrator tells us he's not mad but just nervous and diseased. The narrator can tell us the story calmly.This is telling us that the narrator is not nervous.
The narrator wants to prove his sanity.The narrator was kind to the old man for a week. At 12, he would look in on the old man. He would open the lantern to see if the old man's eye was open. The narrator would ask the old man how he slept. The dead hour of the night, midnight is foreshadowing the time and night the old man will die.
On the 8th night, the narrator wakes the old man up and laughs to himself and continues to walk to the room. The narrator is in the old man's room while the old man is sitting for an hour. The narrator opens lantern and sees the old man's eyes are open. The narrator hears low, dull, quick sound and thinks its the old man's heart. The narrator thinks his heart beat is getting louder. The narrator has new anxiety that the sound would be heard by the neighbors. Death is stalking the old man is personification.
On the 8th night the narrator kills the old man by suffocating him with a mattress. The old man screams to try to get help.
He dismembers the body in the bath tub. The narrator hides the body under the floor board. The police arrive because a neighbor heard a shriek. The police search and the narrator invites in to sit. He starts to hear the old man's heart beat. He flips out and the sounds aree getting louder.
The narrator confesses to the murder because he can't stand the sound of the heart beat. This is symbolizing the narrators own guilt.