The fire can symbolize a couple of things. Yes, it could represent their savagery at the end. But more likely it represents the loss of control. Fire is very powerful. It can be many comforting things, but it can also kill very easily. That's what we have to wonder about the mulberry-birth marked boy who they never saw again--was he killed in that first fire?
That first fire showed how powerful it was and how the boys had to respect it and it would work for them and help to create a signal, along with providing them warmth if needed. However, in the end, there is no control at all. The boys have completely become savages and have all lost their innocence as well. That fire was enough to take down their entire forest, along with any food they might get on the island (pigs and fruit both). They showed no respect for nature or each other. They only wanted Ralph dead. So it could symbolize their savagery, but it also symbolizes the loss of control and total chaos.
For nought so good but strain'e from that fair use/Revolts from true birth, stumbling on abuse:/Virtue itself turns vice, being misapplied;/...In man as well as herbs, grace and rude will;/And where the worser is predominant/Full soon the canker death eats up that plant. [Romeo and Juliet, II,III, 19-30]
Shakespeare's Friar Laurence answers this question: The wildfire exemplifies virtue having turned to vice. It is symbolic of the civilized human nature--a life-saving signal--gone awry. The wildfire destroys the vegetation and life on the island; civilization has "stumbled on abuse."
While the boys are civilized and rational and under control, the fire is tended on a regular schedule. However, when the boys ignore it, the fire, "misapplied," gets out of control. Later, after the boys paint their faces and, hide behind their masks. They act out their savagery and hunt pigs and the fire is untended; thus, it becomes a wildfire and the "canker death eats up that plant": Death and killing take dominance over civilized behavior--"kill the beast! kill the beast!" the boys chant. They become obsessed with destruction, and their obession with killing Ralph allows the wildfire to begin its "canker death" of the island. The wildfires symbolize the virtue of civilization turned to the vice of savagery in the boys of "Lord of the Flies."
The wild fire at the begginning of the story does not symbolize savagery but in fact sybolizes lack of leadership and initiative. Ralph was chosen for chief and he did not really set his priorities straight bacause he wanted to have fun before getting rescued. The fire at the end of the story symbolizes savagery because the savages are trying to kill Ralph but of course they do not suceed.