'A Clockwork Orange'
This is one of those rare cases where I know for a fact the guy making the film didn’t read the book—or at least the final chapter. In the original version of A Clockwork Orange by Anthony Burgess, Alex sees the error of his sociopathic ways and goes on to become a productive member of society, though he does note that his son will likely be just as much of a sociopath as he was. In the U.S. version, however, the final chapter is omitted—the story ends when Alex (having underdone conditioning to rid him of his violent impulses) finds himself once again daydreaming of murder and brutality.
Kubrick based his movie on the U.S. version and didn’t read the original until after he’d already written the screenplay. He never considered using the original ending, believing it to be inconsistent with the rest of the book. Burgess disagreed. “The twenty-first chapter gives the novel the quality of genuine fiction,” he said, “an art founded on the principle that human beings change…The American or Kubrickian Orange is a fable; the British or world one is a novel.”