The Narrator explains why he did it and how he really isn't insane.
Take that, you fiend.
The Narrator didn't like the old man's eye, although he had no interest in revenge or money
What seems to be the problem, officers?
He had stayed there 8 nights staring at it, until with his lantern, he woke up the old man.
OK, I did it! Just make the beating stop!
He then threw his bed onto the floor, suffocating him until he heard nothing but himself.
He then dismembered the corpse, and put his body in the floorboards, and put his blood in the bath. After this, the police come to the door and said what the neighbers heard was him screaming from a nightmare.
After the pressure of guilt, The narrator finally admits to what he did, and ends up going practically insane.