In Chapter 4 of "Lord of the Flies," Jack said they left the fire because they needed meat, but what is the real reason?
The boys left the fire to hunt for meat, but it wasn't so much the actual attainment of meat they were after, but the thrill of the hunt. They followed Jack, who had painted his face, putting on a mask to allow his savagery to emerge more fully, "The mask compelled them." Jack probably does want meat to eat, but more than that, he wants to experience the thrill of hunting and killing. He wants the control that hunting and killing something will bring him, too. He's been at odds with Ralph since the first election when Ralph has chosen as leader and this is one of Jack's attempts to dominate and become leader. Since the boys who were supposed to watch the fire are following Jack's lead, they get caught up in the excitement of hunting and killing as much as Jack does.