How does William Golding's characterization of the boys show a transformation/evolution to complete savages?
There are moments in the beginning of the story where the boys exhibit characteristics of civilized or trained young men who aren't willing to cross certain lines. Jack is unable to stab a young pig with his knife, Roger cannot actually hit the littlun with rocks when he is throwing them near him, and they are all willing to have meetings and submit to the structure that Ralph sets up with Piggy's help.
Soon, these aspects of the boys begin to break down as they become more and more savage. Jack paints his face and then finds himself able to actually kill pigs. Ralph participates in a hunt and is surprised at the pleasure he takes in wounding a pig. Golding shows that all the boys, except perhaps Simon and Piggy, succumb to the more savage and less "human" desires within them.
By the end of the story, the transformation to savage has become complete. Jack is punishing boys who disobey with torture, Roger gleefully pushes a giant boulder onto Piggy and kills him, the hunters take over the island and begin a big hunt to find and kill Roger. Even when the naval officer arrives and the boys are rescued, Ralph weeps bitterly for the knowledge of their true nature and the loss of innocence he witnessed.