In the beginning of the book, Ralph is voted as chief and Jack isn't that thrilled with it and decides to ignore Ralph's commands and go and explore with the rest of the choir.
Jack starts to become uncivilized by becoming more like an animal. The only thing that Jack wants to do now is hunt. Jack realizes that he can make his own tribe away from Ralph and Piggy.
Jack leads his tribe to a march where he's now uncivilized and is becoming a devil in the minds of others. An example would be him instructing a fire dance that leads to Simon being murdered without reason.