Roger, gloomy and sadistic, represents the innate savagery in the boys in William Golding's allegory, "Lord of the Flies." In Chapter Four he is described as having a forbidding unsociable remoteness. When Henry, a small boy, tires of his play and goes to the water's edge, he is fascinated by the tiny creatures that live in the shallow water. As he becomes absorbed in his play, Roger "waited,too." He hides at first behind a great palm, watching Henry. With atavistic behavior, Roger stoops and picks up a stone, "that token of preposterous time," and bounces it to the right of Henry. He misses Henry again and again because
invisible, yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law. Roger's arm was conditioned by a civilization that knew nothing of him and was in ruins.
When Jack appears at a nearby tree, Roger sees him and "a darker shadow crept beneath the swarthiness of his skin...." Like the serpent in the Garden of Eden, Roger represents the intrinsic evil in man. While he is still conditioned by society at this point, Roger does not commit the evil acts that he wishes to do. However, when he has the opportunity later in the novel, Roger, "with a sense of delirious abandonment," hurls against the rocks the boy most representative of rules and order, Piggy. It is also Roger who beats Sam and Eric until they agree to be with the hunters; and, it is Roger who sharpens on both ends the stick that will impale Ralph if the hunters catch him.
Clearly, Roger represents pure savagery, the innate evil in human nature. While he still wears some vestiges of society, he hesitates; however, as he remains on the island and can wear a mask and hunt with Jack, he gives license to his sadistic and heinous acts on a land free from society--a place pristine and unprotected from the "evil that men do" as Marc Antony says in "Julius Caesar."
The episode you are talking about happens in Chapter 4.
By this time, the boys have already started along in the process of losing their civilized ways. However, the part where Roger throws rocks at Henry shows us that they have not yet completely lost their civilized sense of what is right and wrong.
We are told that the reason Roger misses on purpose is because he is still feeling the effects of being civilized. We are told that the old life, the parents and the policemen are surrounding Henry so that Roger can not bring himself to actually hit Henry with the rocks.
Roger is a bully. He is one of the older boys and Jack's friend. When they land crash land on the island there are a few older boys and more littleuns. Henry is the oldest of the littleuns. Roger likes to tease and torment the littleuns. He is also the one later on in the book who kills Piggy by bringing the rock down on him.
Henry is going to the water's edge to play. Roger follows him, and he hides behind a tree. He throws the rocks when Henry is turned towards the water. When Henry turns around he is smiling anticipating that one of his friends is playing with him. Roger ducks behind the tree so that Henry will not see him. He had missed him on purpose to tease him.