Roger is an interesting study because he is constrained primarily by fear of punishment for breaking the rules. Once that fear goes away, such as happens on the island, Roger has no inward mechanism to keep him from sadism.
Golding very carefully sets up the way Roger tests the new boundaries and starts to work out what he can get away with. We learn that Henry is the smallest of the little boys, making him the easiest to pick on. While at first Roger contents himself with kicking apart the castles in the sand the little boys have made, he waits to throw the stones at Henry. Roger only begins to do this when Henry has wandered off to play by himself. Roger also carefully looks around to make sure that older boys like Piggy and Ralph are far away and busy. At this point, he begins to throw the stones at Henry cautiously:
Yet there was a space round Henry, perhaps six yards in diameter, into which he dare not throw.
Ralph knows from past experience that someone in authority might punish him for throwing stones at another boy. Therefore, at this point, he restrains his aggressive impulses: the rules of civilization still operate for him
In this chapter, however, Roger begins, tentatively, to shed civilized norms. He chooses Henry because the boy is small, weak, and can't easily fight back. He is a safe subject on which to experiment. A boy like Roger will quickly revert to savagery once he realizes that he can, but at this point, he does not feel free to do so.