So Mort soons works out that John Shooter is just another part of his personality, Fight Club style, about one hour into the movie after the rest of us. He has already murdered a few people ... and now his wife Amy turns up at the door with the divorce papers.
Inside she finds the place a wreck with the word "SHOOTER" etched everywhere. Upstairs in the house Mort is standing behind a door with the words ... "SHOOT" and "HER" behind him. Very imaginative.
She runs. Outside he stabs her in the leg with a screwdriver as Ted shows up. Mort slams a shovel in his face - and then uses the shovel to finish him off. He then turns to Amy as it fades out, and you can assume she dies too.
Later, he makes his way into town for groceries - but the villagers are scared of him. The police chief calls by his home, recently redecorated, and warns Mort to keep out of town and that he knows he is a murderer, and will take action as soon as they find the bodies.
Mort is calm and goes back to finishing off his latest work. The camera takes us back out "The Secret Window" and, as the intro to the short story told us earlier, his wife is buried outside, just underneath the window, covered by stalks of corn.