Of all of the boys on the island, Roger is most definitely the one guy that I would not want to be stuck with on the island. Simply put, Roger is a sadistic jerk of a kid. He takes pure joy in hurting other people. Readers can see this quite early on in the story and should check chapter four for evidence. Some of the little kids on the island spent some time building sand castles on the beach. When Roger and Maurice come down to the beach for their break from fire tending duties, they immediately set to destroying the castles. That's just cruel.
Roger led the way straight through the castles, kicking them over, burying the flowers, scattering the chosen stones.
What is very interesting about this event is that the text tells us Maurice felt the need to come up with an excuse and vacate the area immediately. He has been conditioned to know that being mean will result in some kind of punishment for him.
In his other life Maurice had received chastisement for filling a younger eye with sand. Now, though there was no parent to let fall a heavy hand, Maurice still felt the unease of wrongdoing. At the back of his mind formed the uncertain outlines of an excuse.
Roger doesn't leave like Maurice does, but Roger still definitely is subject to the behavioral conditioning of his former life. He knows that there are no adults to punish his actions, but he hasn't been in that environment long enough to have truly broken away from the previous societal rules and conditioning. That's why he can't bring himself to truly aim his throws at Henry. He is throwing them near Henry, but he would still be able to claim that he wasn't trying to hit Henry.
Roger stooped, picked up a stone, aimed, and threw it at Henry—threw it to miss. The stone, that token of preposterous time, bounced five yards to Henry’s right and fell in the water. Roger gathered a handful of stones and began to throw them. Yet there was a space round Henry, perhaps six yards in diameter, into which he dare not throw. 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.