Jack Merridew is the real chief in this story, he is the one who has the band of hunters responsible for killing animals for food. Jack immerses himself into the whole persona of chief, he actually becomes very savage in the process.
In the book, Ralph, who is responsible for building the huts in the beginning, has to turn to Jack for food, because he is not a hunter and he and Piggy are starving for something other than fruit.
Jack Merridew is the chief, even if it by default. As he becomes more savage in his behavior, he compels the other boys to follow him. They also engage in vicious behavior that is like that of an angry mob, devoid of reason, driven by the need to attack, destroy, to kill.
Jack exploits his power, using the boys fear of the unknown beast, and also exploits their hunger, both for food and leadership to keep a tight control on activities in the group.
The only thing that stops Jack for killing Ralph is the intervention of an adult, the British Captain who appears at the end of the story.
At the beginning of "Lord of The Flies," the boys all group together when Ralph blows the conch shell. Once the boys all gather round they decide they need a leader. Jack seems to think he should be chief because he is already a leader of the choir. He is arrogant and sure of himself. He wants control, but the rest of the group doesn't agree. The group votes for Ralph as the leader and he accepts the role of "Chief."