What are two significant changes to William Golding's novel Lord of the Flies that are made in the 1990 film adaptation which affect Golding's vision of humanity?
It is a commonly held belief, at least among literary types, that film adaptations are never as good as the books from which they came. The 1990 film version of William Golding's Lord of the Flies is certainly not a faithful representation of Golding's primary theme: when human nature does not have the restraints of society or authority, it will devolve into savagery. While that general theme is still seen in this movie, two elements in this film lessen the impact of what you refer to as Golding's "vision of humanity."
First, there is still an adult on the island. The pilot has lived through the crash and is living in a cave somewhere on the island. Perhaps if none of the boys had known he was there, Golding's point might still have been made effectively. Several of the boys encounter him, however, which completely disrupts the idea that the boys are left without any authority figure after the crash. Ralph's first response when he realizes that none of the adults survived the crash is “No grownups!” It is clear that he is anxious to try living in a world without the order and discipline of adult authority, though he certainly thinks differently later. What a strange turn of events it is to interject an adult into the story, altogether changing Golding's vision.
Second, the boys (and the Marine Corps officer who rescues them) are American, not British. This, too, diminishes the impact of the boys' descent into savagery. British boarding-school boys are perceived, at least, as being more proper and disciplined than their American counterparts, which means their relatively rapid transformation into savages is even more unlikely and therefore more impactful. The Marine Corps officer is not able to say, as the British officer did,
“I should have thought that a pack of British boys—you’re all British, aren’t you?—would have been able to put up a better show than that."
Without question, these two aspects of the 1990 movie might have made the move more appealing to American audiences, but they weakened Golding's primary theme about human nature.