Why does Roger throw rocks to hit Henry?

Roger throws rocks at Henry because he wants to prove he is older and tougher...... to imtimidate him. He stops throwing the rocks because he briefly remembers it's wrong, a bit of civilization still remains within him.

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.