Although Jack doesn’t succeed in killing the pig until chapter 4, his intent is already there beginning in chapter 1 when he, Simon, and Ralph come across a pig caught in the undergrowth. Jack has the perfect opportunity to stab it, but as he hesitates, it gets away. When Ralph asks why he didn’t, Jack makes excuses. But he feels shame about it.
They knew very well why he hadn’t: because of the enormity of the knife descending and cutting into living flesh; because of the unbearable blood (31).
So now Jack feels that he has something to prove.
In chapter 2 we quickly see that killing a pig has become an obsession for Jack. During their meeting after exploring the island, Jack announces that they need an army so they can hunt the pigs. When Simon and Ralph recount how the pig got away before Jack could stab it, “Jack slammed his knife into a [tree] trunk and looked around challengingly,” saying “‘next time!’” (33). His penchant for violence is there on the surface and foreshadows the violence he will soon unleash on the other boys. The pigs are just a stepping stone.
In chapter 3 we see Jack alone in the jungle, crawling through the undergrowth like an animal in pursuit of his prey “and for a minute became less a hunter than a furtive thing, ape-like among the tangle of trees” (49). His other hunters have long since given up, but Jack’s obsession with killing a pig won’t let him quit. He argues with Ralph, justifying all the time he spends on his thus-far fruitless endeavor. “We want meat” quickly becomes “We need meat.”
By chapter 4 Jack has determined that his failure lies in the fact that the pigs see him coming, so he paints his face with colors of earthen clay. When his hunters see his darkly painted face and shrink away in fear, Jack realizes the awesome power the mask gives him.
He began to dance and his laughter became a blood-thirsty snarling . . . and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Under this influence, he is able to actually revel in the blood as he finally kills a pig with his hunters. They deliver the pig to the camp as they chant, “Kill the pig. Cut her throat. Spill her blood” (69). Expecting praise from Ralph, Jack proudly announces, “I cut the pig’s throat . . . There was lashings of blood” (69). From this point on, Jack never washes the mask off. It is this mask that allows Jack’s transformation from hunter to a descent into violent savagery against the boys themselves.