One adaptation I never liked was a movie adaptation of Huxley's "Brave New World" from the early 2000s decade.
In Huxley's dystopia, the main character kills himself at the end because he can't stand the society humanity created for itself, in which essentially no one is an individual, but a blissful, mindless consumer of the "pleasures" offered by the system. A system that reduces any forms of creativity and personal relationships to the most shallow, consumer friendly thing possible, something the main character (a "native" person, grown up into a reservat for the most part of his life) can't stand, after at first being curious about this world of which his biological father was a part of. He kills himself in th eend, because he essentially becomes against his will a "curiousity" for this society and no one leaves him at peace.
The ending was in my opinion very impactful this way, because in its somber and depressing way it was also essentially meant as a wake up call, telling the reader that if they think this sucks, they should do something about it to prevent this world from becoming reality.
In the movie adaptation, the guy actually still becomes a curiousity and is hunted down by some "fans" like he was Lady Di in a french traffic tunnel hunted by reporters. And he dies by accident, when he trips over a stone and then falls down a hill. In doing so essentially stripping the movie of the "impact" his death has in the story, cause while in th ebook it was him being driven to desperation by the society, here it is an accident "society" can't be blamed for as such.
Additionally the movie ends on a scene showing a kid in the society being "adjusted" to the social norms, but indicating that it won't fall in line with what is expected by it, as it drowns out some subliminal messages it is supposed to hear during sleep.
Which again is a "betrayal" to the book, where in this society everyone was essentially from the crib in line with the "norms" because they did never know really better. Something that showed how this world is awful in the way of how huans have turned into something more hollow. the movie however, by ending on the "hopeful" note that something will change, kinda turns the depressing end meant to be a wake up call to the reader into a "bittersweet ending" convincing the viewer that stuff can change without their input and disdain for what this world is.