how has the author prevailed evil in this story?

The island is very much an Eden before the boys show up. The scar (plane crash) is the visible symbol of evil that has come. Evil is then manifested in various ways. The hunts become more brutal and the antagonism becomes more violent. The idea of a beast lurking in the jungle is a manifestation of their own fears, their own darkness. Simon attempts to explain that all that scares them is inside of each boy. This explanation, of course, is lost on the boys. The Lord of the Flies or Beelzebub is manifested (sorry I keep using that word) by the pigs head on a stick. It represents the darkness of man's heart that they cannot run away from. How can you run away from yourself? In the end most of the boys have turned their darkness, their evil. Self-destruction is inevitable. With the death of Simon evil is left to flourish in all its splendid awful glory.

isn't the fire "out of control" another evil as well? i think there are more..

The fire itself isn't evil. The fire in Ch. 2 is perhaps a metaphor of the chaos and destruction that these boys are capable of. The boy's simply unleash something they can not control. One of Golding's themes is that the only thing on the island that is evil are the boys themselves. Everything else, including the fire, are personifications.