When/why does Piggy try to get Ralph to leave Jack's group in "The Lord of the Flies"?
At the end of Chapter 6 of "The Lord of the Flies," the boys decide that they will search for the beast in the only unexplored area of the island; after their search, they will relight the fire. Then, in Chapter 7, Jack leads the boys on a pig-run and Ralph contentedly follows. When Roger calls Jack to inspect pig droppings, the boys pursue a boar and Ralph, who is charged by a boar, throughs his spear, striking the pig in the nose. As the boys pursue this boar, they notice Ralph's excitement at hitting the pig. In the break of their losing the pig, Ralph's boasts of his striking the boar. Clearly, he is caught up in Jack's world of hunting.
They decide to look for the beast and reach the shoulder of the mountain. Jack suggests that they "creep forward on hands and knees" to glimpse the beast who may be asleep. Roger and Ralph move forward, leaving Jack behind, but he whispers accusingly to Ralph "Scared?" Ralph comes upon "the ruin of a face" and takes "giant strides" back to camp.
In camp Piggy questions Ralph about the existence of the beast." Jack argues with Ralph who contends that "I don't think we'd ever fight a thing that size." They argue and Jack leaves. Piggy is "indignant" and scolds Ralph for having called him a coward and for letting Jack be defiant by saying that he is "not going to play any longer. Not with you." He tells Ralph, "We can do without Jack Merridew."
The rational Piggy understands that Jack is a force to be reckoned with, not joined. He is insulting to both Ralph and Jack, undermining their authority; also, his cruel acts against the little'uns that he has pretend to be pigs is threateningly sadistic.