In the novel, there are many examples of good versus evil. Golding inserts in the novel many Christian symbols--the hit-you-over-the-head kind. For example, Simon is the Jesus figure: he is kind, works on the huts (Jesus was a carpenter), feeds the children fruit that they cannot reach (Jesus fed people in the Bible), and he goes to a private place with natural candles on the trees to meditate (similar to going to a church to pray). Simon is also the one who has the standoff with the beast--the boars head on the stick. This is where the beast informs Simon that there is no beast to be afraid of: they only have to be afraid of themselves--the beast within.
Furthermore, many experts believe that Jesus had epilepsy (a seizure disorder), and Simon has this condition, too. He has a seizure at the end of his conversation with the beast, which is why he has so much trouble walking down the mountain after he wakes up. He slinks down the mountain, hunched over, and the kids (at first) mistake him for the Beast. But then they know who it is and they kill him anyway--good versus evil again. As his body floats away, the head is surrounded by bioluminescent creatures that make a "halo" around it.
Then there is Jack versus Ralph. Jack creates a tribe, one that sacrifices animal parts to the Beast--very reminiscent of young civilizations. Maybe the Beast will be pleased and won't hurt them if they sacrifice these pigs heads. They have moved from science to superstition. Jack's henchmen, Roger and Maurice, are as evil as they come. Roger tortures kids, and he eventually kills Piggy with a large boulder.
Jack, at the end, reverts to human sacrifice, with a stick "sharpened at both ends" for Ralph, as he plans on sacrificing Ralph's head to the Beast. Then Jack's entire tribe hunts Ralph until they are rescued by the adults. All of the kids, with the exception of Simon, have let their inner beast out on the island at some point.