What are the examples of Jack's becaming more primitive and animalistic?
Jack, the former choirboy, is becoming more savage. This is evident in his forming another group when he cannot be the leader. His group masks their faces and paint their bodies to become a roving mob looking for violence without consequence.
Jack is credited with the enforced membership of Sam and Eric to his group. His method was not described but it was accepted that it included pain to get them to join. His plans to rule include violence in an increasing rate.