In Chapter 4, how did the mask affect Roger's behavior? Why is the mask “a thing of its own”?

In Chapter Four, Maurice and Roger emerge from the forest and, seeing two of the small boys, the bigger boys destroy their creations in the sand. Shortly thereafter, Roger throws rocks that just miss Henry as he plays on the beach. But, when Jack calls him and displays his mask, Roger "understands" that there is something compelling about the mask.

At the beginning of the chapter, Roger and Maurice destroy the sand castles the littluns...

In Chapter Four, Maurice and Roger emerge from the forest and, seeing two of the small boys, the bigger boys destroy their creations in the sand. Shortly thereafter, Roger throws rocks that just miss Henry as he plays on the beach. But, when Jack calls him and displays his mask, Roger "understands" that there is something compelling about the mask.

At the beginning of the chapter, Roger and Maurice destroy the sand castles the littluns have built, causing Percival to get sand in his eyes and cry. Later, Roger watches Henry play in the pool and he throws stones around Henry "...because his [Roger’s] arm was conditioned by a civilization that knew nothing of him and was in ruins" (Ch.4).

Roger has been punished in school for harming others. His society has conditioned him with its values. However, after Henry loses interest and leaves the beach, Jack, who has been watching Roger, eagerly calls the mean boy over to him. Jack explains that when they hunt, the pigs see his face and run off. But, if he disguises himself, then he may be more successful in killing the pigs.

He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing of its own, behind which Jack hid, liberated from shame and self-consciousness. (Ch. 4)

Watching Jack, Roger understands that Jack acts out his natural savage urges when he can hide behind his mask. The mask becomes "a thing of its own" as it liberates those who wear one from the conditioning of their society, and it compels the others to obey. The constraints of society are what has kept Roger from hitting Henry with the stones that he has thrown. Later, when he wears a mask, Roger also liberates his own cruel tendencies and acts upon them with great violence.