What rationalization do Ralph and Piggy arrive at concerning their role in Simon’s death?

That it was a mistake. Piggy tries to say it was an accident.

Piggy tells Ralph that Simon's death was an accident even though Ralph feels the weight of resopnsibility and goes so far as to say it was murder. Piggy talks Ralph into accepting the idea that it was an accident because the boys in the wild dance created a lot of confusion and the thunder and rain added to the darkness and confusion. Ralph and Piggy agree that they were both on the outside of the circle and couldn't really see what was going on. Because they could not see what was going on and the wild dance created so much confusion, they persuade themselves that Simon's death was an accident that they could not have prevented.