Roger's society shaped the way he behaves because he'd learned that if he didn't behave properly, he'd be punished. He'd learned, probably from experience, that if he acted out the way he wanted to, that if he responded to the bully and evilness within himself, his parents, his school, and even the law would come down on him. The first sentence of the quote attests to that. By saying that the protection of parents, the school, the policemen, and the law sourrounded Henry (the child that Roger was throwing stones at), the indication is that Roger had learned that these forces were between doing what he wanted to do and doing what he knew he could or could not do without fear of punishment. The second sentence of the quote is the more frightening sentence. The words, "...a civilization that knew nothing of him..." hints that Roger is a sociopath or worse. The society did not know of his innermost desires, of his evilness inside because he'd learned to hide it so that he could exist in that society. The final words, "...and was in ruins,' is the most telling part of the quote because it lets us know that the society that kept Roger at bay no longer exists and therefore the evil within him is free to come out. And it does come out. Roger becomes the most evil person on the island because he seems to act without conscience. By the end of chapter 11, after he has crushed Piggy, he wields "a nameless authority" because he does not care that he killed Piggy. Now that he is free of the society that shaped him, he is free to act the way he's always wanted to act.