In chapter eight, the boys act out the pig killings as an opportunity for role play--in effect, it is their way of emotionally dealing with the violence of killing the sow. By making light of it and cracking jokes, the boys' diffuse the intensity of the moment.
"This time Robert and Maurice acted the two parts; and Maurice's acting of the pig's efforts to avoid the advancing spear was so funny that the boys cried with laughter" (136).
They do not know how to handle the enormity of what they had just done, but by play acting the killing, the boys are able to translate a very violent experience into something they can experience safely and humorously.
Seen from a different perspective their levity in the situation also reveals the boys' descent into savagery, making something comic of a situation that should have been a serious moment.