The forces of good and evil are generally divided up into the forces within the boys. Piggy and Ralph are trying hard to help the boys govern themselves and to organize to help effect a rescue and to make sure people are fed and accounted for.
Simon, who also serves as the Christ figure, is aligned with no one, shows particular care for the little ones, and eventually has the epiphany and understands that the evil forces are really those within the boys, the beast is within. Soon after this realization he is killed, of course it is also notable that his vision comes after wandering off alone into the wilderness.
Jack and to an even larger extent Roger represent the evil within boys. Particularly Roger as he reaches the point where he has no issue with killing and does it with glee, he really represents evil by the end of the story.