If you remember when the head of the pig is speared and put on the stick, this is a reference back to their first sacrifice. The Roger and Jack began by killing the pig to sacrifice for the beast. The pig's head represents the innate savagery that the boys have been overcome by.
Now in chapter 12, with the reference of the double sharpened edged spear, Roger and Jack have moved on to murder, to human sacrifice. Roger says "in hushed tones" that he will use a two sided sharpened spear indicates he hopes to kill Ralph. Then he wil put Ralph's head on the sharp end and stab the other end into the ground. When thinking about murder, we are horrified by the violence and the blood that is implied by killing a young boy with a spear. But Roger wants to go as far as to then show off his power, his skill by putting the head on a spear.This further suggests the boys descent into savagery - specifically Roger and Jack.