Ralph and Piggy disclaim their parts in what happened to Simon by saying that they joined in near the end, it was an accident, and that they were in the outside circle.
In chapter 9 of "Lord of the Flies," Ralph and Piggy disclaim their parts in Simon's death by suggesting that they didn't participate in the killing intentionally and by arguing that they were caught up in the frenzy of the moment. They claim that they were swept away by the group's madness and didn't mean for Simon to be harmed, shifting the blame to the collective actions of the group.