Jack is not a very nice character when when we first meet him in William Golding's Lord of the Flies, and he does not get any nicer over the course of the novel; in fact, he transforms from a rather heartless, unkind boy into a cruel murderer.
Jack is the head of a choir who is completely intimidated by him; they do not sit until he tells them to and they grudgingly vote for him only after Jack wills them to do so. He is the only boy on the island who had a knife with him when the plane crashed, and he is cruel to and dismissive of Piggy just because of how he looks.
Soon Jack becomes obsessed with hunting, tirelessly walking pig trails and sniffing their droppings. It is when he finally paints his face, hoping to disguise himself from the pigs, that a major transformation takes place.
He looked in astonishment, no longer at himself but at an awesome stranger.... He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. The face of red and white and black swung through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell silent and blundered away through the bushes.
The other boy is appalled and frightened by what he sees, and it is not just the paint on Jack's face which causes such a reaction. Something changes in Jack when he is able to hide behind a mask; he no longer feels and shame or self-consciousness, so he can do whatever he chooses without remorse--and he does.
By the end of the novel, Jack has moved from hunting and killing pigs to hunting and killing humans.
Jack made a rush and stabbed at Ralph’s chest with his spear.
When Ralph escapes, Jack (now referred to as "the chief") orders his tribe of savages to find and kill Ralph. If a rescue ship had not arrived, they would have done it and Jack would have been thrilled.
Despite these things, Jack is still just a little boy, which makes his transformation even more appalling. When the naval officer looks at Jack, he sees this:
A little boy who wore the remains of an extraordinary black cap on his red hair and who carried the remains of a pair of spectacles at his waist.
Of course this is Golding's way of reminding us that it is the nature of man to revert to savagery when he has nothing to restrain him, and Jack is the character he uses to demonstrate this theme.