The bigger boys go with Jack because they want to be part of the hunt and they were former choir boys who think they owe allegiance to Jack. They want to show their bravery in the face of "the beast" and get a chance to experience the death of the pig. When Jack says, "sharpen a stick at both ends," he is getting ready to mount the Pig's head on a stick as an offering for the beast. One end of the stick will be in the ground and one will be in the pig's head. This shows how savage the boys, especially Jack, have become and sets up the "conversation" between Simon and the "Lord of the Flies".