The primary theme of Golding's novel was civilization versus savagery--what would happen to people (in this case, young boys) of high society when stranded in a situation which requires them to survive. The main concept is that humans will do what is necessary, that martyrdom is not typical of the human condition. Only those that are not strong enough to lead do not become savages, and Golding proves this in the deaths of specific characters.
In other words, Golding's point was that even the most moral of humans will regress into a "savage" when given no other option. With no adults, no government, and no unity on the island, Ralph was forced into a very specific role. Jack's wilder, more murderous group initially shocks readers; the greater shock comes from Ralph's change, due to our belief in an inherent goodness and the expectation of a "happy" ending.