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Behind them on the grass the headless and paunched body of a sow lay where they had dropped it. (11.129)
At this point, we get the feeling that the boys aren't even bothering to eat the meat they're killing. They're killing for the fun of it—something that Golding unequivocally identifies as "savage."
Ralph launched himself like a cat; stabbed, snarling, with the spear, and the savage doubled up. (12.165)
Who's savage now? When his life's at stake, Ralph can be as primitive as anyone else—like all of us. Some people just take longer to get there than others.
The sticks fell and the mouth of the new circle crunched and screamed. The beast was on its knees in the center, its arms folded over its face. It was crying out against the abominable noise something about a body on the hill. The beast struggled forward, broke the ring and fell over the steep edge of the rock to the sand by the water. At once the crowd surged after it, poured down the rock, leapt on to the beast, screamed, struck, bit, tore. There were no words, and no movements but the tearing of teeth and claws. (9.89-99)
This isn't a cluster of boys ganging up on another one; it's a battle between a beast and a "circle," a "crowd," and a "ring," with "a mouth" and "teeth and claws." Pretty brutal.