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Jack’s Background:

I think that Jack is a naturally power-hungry person, He always seems like he wants to be in control and have attention. With the addition of Ralph into the picture and Ralph being elected leader, this angers him and eventually his human nature (which is to be savage) took hold. 

Effect on Jack:    

I think that when Jack was not even considered for the leadership position he became savage in an instant. He immediately appointed himself as head hunter which give him some leadership. When he started disagreeing with Ralph later in the book, he wanted things to change for his benefit, leadership wise.He realized that he was the leader of the group of people who like  to kill and who had the weapons. He changed from a choir member to a savage leader when faced with an unpleasant situation without parental guidance. The old constraints put on them by those in authority faded over time in this new environment.