Comparatively he is. Nicholson’s Jack is full of suppressed rage, his entire character is one you’re not comfortable with. I’m not sure his meltdown is any more ambiguous than in the book: supernatural forces make him go nuts. The difference in the book is that it has the luxury as books do of giving you a much fuller sense of his character and the decisions he’s made that aren’t good, etc.
I’m a fan of Stephen King when he’s good. I read The Shining and Salem’s Lot back when I was around 11 or 12 I guess and thought they were both excellent. When I re-read them in the last 10 years Salem’s Lot was okay but his writing seemed far less good as an adult, whereas I thought The Shining still was a really strong and excellent read.
(Would still probably say his best book is Dolores Claiborne, FWIW, but I’ve not seen the film of that one.)