why is the killing of the sow described in so much detail?

This is a shocking scene. I still don't like reading it. Golding uses rape imagery to describe the kill. If you examine it again, you will see. The boys are not content to simply kill the sow. They abuse it with brutal sexual undertones before the savegely slaughter it. Rape is more about power than anything else. It is not enough for them to simply kill the sow and impose their will with its life; the boys whip themselves into a socio-pathic frenzy and mutilate this sow in uspeakable ways.