The boys arrive on the island, realize that no adults are present, and start organizing. Ralph is elected chief, but Jack takes over the group of hunters. The seeds of conflict are sown.
Jack is obsessed with killing a pig for them to eat. Ralph is obsessed with keeping the signal fire going in. Everything devolves into chaos and anarchy: Piggy's glasses are half-broken; Jack and his hunters kill a mother pig in a gory rape/murder scene, leaving its head impaled on a stake as a gift for the beast; Simon names the pig's head "the Lord of the Flies" and has a vision in which it speaks to him.
Basically, we're about as far from Ralph's vision of a deserted island Congress as we can get.
When Jack pulls most the boys away from Ralph onto "his side," they become more and more violent and savage—so savage that they beat Simon to death in a frenzy of violence before murdering Piggy and chasing Ralph through the jungle… right into the arms of the British Navy.