In the first chapter of the novel, Jack has an opportunity to kill a trapped piglet on his trip with Ralph and Simon to the mountain to get the lay of the island. In this scene, Jack captures a piglet tangled in creeper vines, but at the last moment, is unable to slit its throat with his pocket knife. The enormity of flesh and blood becomes too much for him, and Jack feels ashamed in front of the other boys who witnessed this moment of weakness.
From that point on, Golding characterizes Jack as a boy who has something to prove to the others in the tribe. He pushes himself to hunt, to prove to himself that he can overcome his aversion to blood, and in doing so, cancel out any signs of former weakness. The scene with the piglet is key to understanding Jack's character, because the reader, like the other boys, bears witness to his weakness, his frailty; all of Jack's later obsession with hunting and bloodsport is his way of over-compensating for his failure to kill the piglet in the first place.