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…The ground was hardened by an accustomed tread and as Jack rose to his full height he heard something moving on it. He swung back his right arm and hurled the spear with all his strength. (3.5)
We don't know anything about Jack's training, but we're guessing he didn't have much chance to practice hurling spears when he was busy singing C-sharps. It sounds here like he's just a natural: you can take the boy out of the jungle, but you can't take the jungle-beast-killing-prowess out of the boy.
[Jack] tried to convey the compulsion to track down and kill that was swallowing him up.
"I went on. I thought, by myself—"
The madness came into his eyes again.
"I thought I might kill." (3.37-40)
You say pot-ay-to; we say po-tah-toe. You say this is Jack's real nature, subdued by culture; we say that the island is eroding his true self. (Or the other way around; we haven't actually made up our minds.) What does Golding seem to think?
Then, amid the roar of bees in the afternoon sunlight, Simon found for [the littluns] the fruit they could not reach, pulled off the choicest from up in the foliage, passed them back down to the endless, outstretched hands. (3.138)
Talk about innocent: Simon is the only one who bothers helping the littluns out, totally disregarding all the savage power struggles going on behind his back. (Also, notice the difference between Simon innocently picking fruit—how Edenic—and Jack killing a boar?)