Roger, who is the most sadistic boy on the island throws the stones around and near the boys because there is in him still enough conditioning from his British society to prevent him from harming anyone:
In Chapter 4 of "The Lord of the Flies," it is Roger who leads the way from the forest to the water for a swim. He kicks over the sand castles that the boys have built, "burying the flowers, scattering the chosen stones." He is described as possessing an "unsociable remoteness" which could turn to something "forbidding." As Henry sits on the sand near the water's edge, he tries to control the little creatures that come to feed at the edge. Henry makes little runnels that the tide fills in his effort to exerice control. As Roger watches, several nuts fall around him. He looks "from the nuts to Henry and back again." Then, Roger stoops to pick up a stone, aims, and throws it at Henry "to miss." The stone,
that token of preposterous time, bounced five yeards to Henry's right and fell in the water.
Roger throws more stones, but all go around Henry.
Here, 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 Henry looks around to see from where the stones come, Roger whips behind the palm tree, "breathing quickly, his eyelid fluttering." It is as though Roger expects some punishment.
As the boys stay longer and longer on the island,the guilt and conditioning of his society leave Roger and he gives in to his sadistic nature. Like Jack who masks himself, Roger will be "liberated from shame and self-consciousness" that are still in him from his conditioning.