What the hunters do in Chapter 4 to prepare for a second hunt is to decorate themselves. Specifically, they paint their faces into pretty hideous masks. Jack is the one who gets this started. He says at first that he is doing it for camouflage, to keep the pigs from seeing his pink face.
After he gets himself painted up, Jack leads the hunters in a bit of a dance while Jack talks in a bloodthirsty way about stabbing the pig.
By doing these things, Jack is showing that he is moving more towards savagery.
In the book The Lord of the Flies Roger is somewhat of a sadist. He torments the younger children. The situation that you are discussing was when Roger had followed Henry to the water. Henry had bent down and Roger hid behind a tree. He threw rocks at Henry. He missed him on purpose, but stayed hidden. Henry thought that the other children were throwing the rocks.
Roger is also the one who kills Piggy. He throws the rock that causes Piggy to fall off of the cliff. He has lost all evidence of his civilized self. In the end of the book, he uses a long stick as a lever to launch a rock at Ralph's hideout. He doesn't care if he kills him or scares him out of his shelter.