The fact that he changed the endings of a lot of what he adapted is very significant I agree. Steven King hated Kubrick's adaptation of The Shining - the ending especially is much darker than the book, which is already a horror novel. And in the book A Clockwork Orange by Anthony Burgess, the ending is completely different. Alex just grows out of his violent phase and mellows out. In the film, it's much more cynical and creepy with a ginning Alex "restored" to his original psychopathic condition by the corrupted "state".