True enough to the book.
It has been decades since I've read the book, and also since I've seen the movie.
For the few who are unaware, it's about the savage ways that a group of young boys take on when left on their own after a plane crash.
One of the boys becomes a sort of "anti hero" just by not being a sadist. He begins by being a leader, but some of the other boys begin being sadistic savages.
It's a bit of a reflection on the debate long ago between Voltaire and Rousseau, about the nature of man. The French Revolution and other incidents bore out Voltaire's cynicism and mocked Rousseau's optimism.
Other movies have the same motif. Anzio, Ulzana's Raid, End of the Spear, while which reflect on the need to understand that we have to fight inner demons.
Here, two boys totally give in to the demons, and two boys totally reject the demons. Other boys fall in between, but find that if they reject the demons, they must do so in an underground movement.,
It does make for some characters whose motivation is just pure evil, because they are totally possessed by demons. It's something everyone must fight against all the time.
A film to compare this with is "Fortress", which is much the better film, because there is an adult who mentors the young boys and girls who go through an ordeal, and in "Fortress" we see them work together for a common good to help every individual.
Which is the exact opposite of what "Lord of the Flies" does. The boys here do the exact opposite. It's depressing, but not contrived IMO. It shows how ugly that ugliness truly is, which is its saving grace.