Jack Merridew is one of the four primary characters in Lord of the Flies by William Golding, and he is the driving force behind the decline of civilization on the island.
One adjective which describes Jack is controlling, When Jack and his choir arrive at the meeting place to join the other boys, Jack does not allow them to sit until they finally beg him and he relents. Though they are hot in the tropical sun, Jack does not allow them to take off their hats and cloaks (their choir uniforms) until he is ready to do so. Being controlling is a consistent characteristic in every interaction Jack has with every boy on the island. Though at times he and Ralph get along reasonably, Jack is constantly on edge and ready to pick a fight when he senses anyone is trying to challenge his self-appointed authority. As chief of his tribe, he rules through intimidation and control.
Another adjective which suits Jack is selfish. He does not like how Piggy looks, so he ignores the rule that whoever has the conch gets to talk uninterrupted. He does not see any usefulness in the candle buds Simon finds, so he slashes at them with his knife. He wants meat and wants to hunt (obsessively), so he does nothing to help make the shelters or maintain the camp. He is eager to get rid of anything that keeps him from getting what he wants, so he does what he must to eliminate any impediments.
Finally, the word which best describes Jack is savage. He is the only one of the boys who had a knife on the plane with him, and he constantly brags about how he is going to kill a pig long before he actually does it. Jack's final transition into savagery begins when he paints his face to hunt.
He looked in astonishment, no longer at himself but at an awesome stranger. He...leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Without any authority, any shame, or any conscience, Jack is free to act out his full savagery--which of course he does.