For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Community Organizer
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.
Act II
Mmmm, Bacon
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.
Act III
The Savage in the Mirror
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.