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[Jack] began to dance and his laughter became a bloodthirsty snarling. (4.33)
Jack is taking the whole "becoming one with your prey" thing a bit too literally. Here's he's practically morphing into an animal, with the kind of "bloodthirsty snarling" you'd associate with a man-eating tiger rather than a 12-year-old choir boy.
He [Jack] capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. (4.33)
Well, that's one way to answer the question. If Jack is hiding behind the mask, then the thing/person/creature committing these heinous acts isn't Jack; it's the mask. Is Golding giving Jack a way out?
The hunters' thoughts were crowded with memories […] of the knowledge […] that they had outwitted a living thing, imposed their will upon it, taken away its life like a long satisfying drink. (4.121)
There's more here than a simple survivalist instinct to kill for food. The boys aren't hunting just because they're hungry; they're hunting because they need the power. Hm. "Imposed their will upon it," "taken away its life"—that sounds a lot like war to us.