What does the dead pig symbolize and why would Golding choose such a graphic image

This is the physical manifestation of the Devil or Beelzebub. It represents the darkness that the boys have embraced through the killing and torturing of the sow. Simon, our Christ figure, finally comes face to face with it. Check this out,

http://www.gradesaver.com/lord-of-the-flies/q-and-a/what-is-the-lord-of-the-flies-who-names-it-61291/

http://www.gradesaver.com/lord-of-the-flies/q-and-a/what-is-the-lord-of-the-flies-who-names-it-61291/