Lord of the Flies, by William Golding, is a novel which explores the effects of living in a world without laws or authority. Jack is an English schoolboy who was the rather feared head of the choirboys, and he is the one boy in the entire group who had a knife.
When Jack, Ralph, and Simon go exploring at the top of the mountain, Jack talks and acts much more aggressively than the other two, and he boasts that he will lead his hunters to provide food by killing pigs. When he has the chance to do just that, he does not act. He tries to justify his failure to act, but Ralph and Simon (and undoubtedly Jack himself)
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.
The boys are all still living under the constraints of civilization, and killing in such a violent manner is just not acceptable behavior.
By the middle of the novel, though, Jack has begun painting his face in order to hunt, and this "mask" allows him to ignore any objections to killing. In fact, he is proud of each of his killings and seems to grow more eager to kill with every conquest.
Finally, in the final chapters of the story, Jack is responsible for brutally killing Simon and Piggy. He no longer has any pangs of guilt regarding such bloody acts, and as the story ends he has enlisted every boy on the island to hunt and kill Ralph.
Golding's primary theme, which suggests that living without restraints and authority leads to destruction, is best represented by Jack's transition from choirboy to murderer.