I ran into a copy of "Four Past Midnight," the anthology by Stephen King that contained "Secret Window, Secret Garden" which inspired the movie of the shorter name. I just had to flip through it to find the answer to something that's been bugging me since I saw it: was that the original ending?
Answer: No. In the short story, the "hero" is indeed John Shooter, the product of his deranged imagination. But in the original ending, he is killed and his wife is the one who figures out and explains the twisted origin of Shooter. Which leads me to the question, why the heck did they change it? I had a huge problem with turning the hero into a villain who gets away with a grisly and unnecessary murder. They even seemed to be apologizing for it by establishing that he didn't get away with it, that the sheriff and everyone in town knows he's done it. So why not just leave it the way it was? And what was the obsession with corn from?
Screenwriters...they just can't leave well-enough alone.