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[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.
They surrounded the covert but the sow got away with the sting of another spear in her flank. The trailing butts hindered her and the sharp, cross-cut points were a torment. She blundered into a tree, forcing a spear still deeper; and after that any of the hunters could follow her easily by the drops of vivid blood […].
Here, struck down by the heat, the sow fell and the hunters hurled themselves at her. This dreadful eruption from an unknown world made her frantic; she squealed and bucked and the air was full of sweat and noise and blood and terror […]. The spear moved forward inch by inch and the terrified squealing became a high-pitched scream. Then Jack found the throat and the hot blood spouted over his hands. The sow collapsed under them […].
At last the immediacy of the kill subsided. The boys drew back, and Jack stood up, holding out his hands.
He giggled and flecked them while the boys laughed at his reeking palms. Then Jack grabbed Maurice and rubbed the stuff over his cheeks . . .
“Right up her ass!” (8.191-196)
Get all the kids out of the room, because this has just gone from understandable food-related slaughter to... something else. The hunt is no longer about just having meat to eat—it's about literally bathing in their power over a helpless animal. We're not surprised that people tend to read this as a rape scene.