How does Roger's change lead to Piggy's death?

Roger is the boy who, from the very beginning of the story, tests the boundaries of what depths of savagery are acceptable. In chapter four it is Roger who begins to push at the limits of sociopathic behavior by tormenting Percival, Henry and Johnny and then following Henry and throwing rocks at him. He can't quite bring himself to hit Henry yet but that quickly changes.

Roger's descent into violence and savagery is displayed again when he tortures the sow to death by ramming a sharpened stick into its anus; he clearly revels in the horrific pain the animal feels. This progression into a truly violent and sadistic state is important as it leads to Piggy's death.

There is no particular reason why Roger pushes the boulder off the cliff to fall and crush Piggy, other than his lust for violence and blood. The murder of Piggy marks Roger breaking the last boundary of society.