The narrator is hearing things in the heaven, earth, and hell. He has a mental illness. He hears things out of the ordinary.
The narrator is bothered by the old man's eye and wants/plans to kill him.
What was that shriek? !
For seven nights the narrator would look at the old man sleeping. He would open the door and peek his head in and use his lantern to see if his eye was open. The sound of the lantern made the old man wake up, but he didn't move because he was scared. The old man's eye made his blood run cold and was like a vulture.
I did it I admit that I have killed the old man .
On the eighth night, the eye was open. The narrator kills the old man by soffacating him with his bed. He is violent, and insane.
After dismembering the old man's body, the narrator is questioned by two officers about a shriek they have heard.. He seems sane and says that the voice was his dream.
The narrator's guilt made him hear the sound of the old man's hideous heart beating. He confesses to his deed that he killed the man.