The theme of savagery versus civilization in "Lord of the Flies" captures the competing impulses inside humanity: the first instinct is that of civilization, where individuals follow rules created for the good of the group, and they possess moral values, versus the second instinct toward violence, evil actions, selfishness and power at any cost. When looked at from a broader lens, this is the innate conflict of good versus evil.
Golding associated evil with savagery and good with civilization. In his novel, the characters are well-trained boys from the civilized country of Great Britain who are lost in a wild jungle. Slowly, they lose their morals and give in to their baser desires for power, control and violence, which Golding figuratively depicts through the beast.
Golding believed that civilization merely masks the beast within men. When Jack and his tribe become savages, they begin to worship this beast they have imagined, even leaving it offerings. Golding's characters embody these competing instincts of civilization and savagery: Ralph represents order and civilized leadership, while Jack represents anarchy, barbarism and a thirst for power.
At the core of their most base and innate desires, Golding believed people were evil. Civilization suppresses the beastly desires; savagery exploits them and enables people to give into their desires for violence and evil.