Lord of the Flies, by William Golding, is a simple story about some English schoolboys who have been stranded on a tropical island without any adults after a plane crash. What follows is what Golding calls his "attempt to trace the defects of society back to the defects of human nature."
The boys soon loosely divide into two groups, led by two of the older boys, Jack and Ralph. Jack is obsessed with hunting, and he and his group pay little attention to anything else. Ralph is concerned about keeping a rescue fire lit so they will have a chance to be rescued, but no one else seems too concerned about it. At least one ship passes by without noticing the boys on the island.
Soon things on the island devolve into chaos and savagery. Jack and his tribe are consumed with hunting and violence; Ralph and his few followers are unable to defend themselves against the savagery. Things begin to change when Jack starts painting his face to be a more successful hunter.
He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Without the restraints of society (shame) , of authority (in the form of adults), or his own conscience, Jack is free to pursue whatever evil he has in his heart--and he does.
Several boys are murdered (Simon and Piggy) and Jack soon controls every boy on the island but Ralph. If a naval commander had not come to rescue them (ironically because of the conflagration Jack and his savages lit to flush Ralph out of hiding so they could kill him), the savagery would have intensified and none of the boys would have survived.
Golding graphically develops his theme, that the heart of man (his human nature) is not naturally good but instead will seek to satisfy itself however it chooses--and it will inevitably choose evil.