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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?
[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.