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He [Jack] began to dance and his laughter became a bloodthirsty snarling. (4.33)
The boys’ dancing is reminiscent of religious rites.
There was a space round Henry, perhaps six yards in diameter, into which [Roger] dare not throw. Here, invisible yet strong, was the taboo of the old life. (4.14)
"The old life" is civilization. Roger is losing it, but he isn't quite gone. Civilization and all its rules are still holding him back—for now. In a few more chapters, it'll be all savage, all the time.