What act of cruelty does the boys do upon one another?

Well, pretty much take your pick from a prolific list! THe humiliate Piggy, kill Simon (their Christ figure), Roger tortures Wilfred, Kill Piggy......

well in the novel in the beginning evry thing was fine until jack wanted to take over from ralph.then thats when the team of hatred came in,jack left and formed is crew which was called coastal rock taking piggy along then ppiggy and ralph decided to get it on gettin it piggy was killed by roger with a rock,another one is that simon was also killed by the littluns and biguns thinking that simon was he beastie that torturing was torturing them