Why does Roger throw rocks at Henry?

In Lord of the Flies, Roger throws rocks at Henry because he wants to scare and dominate a weaker child, while the habits of civilization still keep him from actually hitting him.

The scene comes after Henry, one of the "littleuns," plays at the edge of the ocean and enjoys controlling the tiny animals there. Roger watches him from a distance, then begins throwing things near him. The crucial point is that Roger’s cruelty is already present but still partly restrained: “Roger stooped, picked up a stone, aimed, and threw it at Henry—threw it to miss.” He is testing the pleasure of frightening someone smaller than himself, much as Henry has been enjoying his own small power over the shore creatures.

Roger’s aim shows the limit he has not yet crossed. He throws more stones, but “there was a space round Henry, perhaps six yards in diameter, into which he dare not throw.” That invisible space is the lingering force of the world the boys came from: “Round the squatting child was the protection of parents and school and policemen and the law.” Roger has no adult nearby to stop him, yet he still carries the old rules inside him.

The scene becomes relevant later, when Roger becomes one of the clearest examples of savagery released from restraint. Early on, he can only pretend to hurt Henry, but later, he rolls the boulder that kills Piggy and helps enforce Jack’s rule through terror. His throwing rocks is an early stage in the boys' gradual transition to openly hurting and killing one another.