William Golding's Lord of the Flies is a novel which is set on a deserted island; the characters are proper English schoolboys who are left there, without any adults, after a plane crash. Throughout the novel, Golding traces their deterioration from civilized boys into savages. What happens in chapter eight is just another picture of the depravity into which so many of them have fallen.
The hunters are following a sow, and initially they wound her and she runs. When they finally do kill her, the scene is described, as you say, in "vivid and gory detail." In fact, the language of the description suggests a violent rape scene.
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. Roger ran round the heap, prodding with his spear whenever pigﬂesh appeared. Jack was on top of the sow, stabbing downward with his knife. Roger found a lodgment for his point and began to push till he was leaning with his whole weight. The spear moved forward inch by inch and the terriﬁed squealing became a highpitched scream. Then Jack found the throat and the hot blood spouted over his hands. The sow collapsed under them and they were heavy and fulﬁlled upon her.
This is just another way Golding demonstrates the deterioration of civilized behavior among the boys on this island when they have no authority or restraints to check them.