At first, fire was a symbol of civilization. Ironically, the fire that lured the ship to the island was not a signal fire, but rather a fire of savagery intended to kill Ralph.
When the boys first arrived on the island, they were well-behaved orderly children longing for rescue. They created rules with the conch and the signal fire.
As time progresses, some of the boys led by Jack naturally lost their sense of innocence and transformed into bloodthirsty hunters. Only Ralph, Simon, and Piggy were capable of understanding rules of civilization.