Jack Merridew is depicted as a cruel, aggressive leader, who undermines Ralph's authority and establishes his own tribe of savages on the opposite end of the island. There are numerous adjectives to describe Jack, which emphasize his bloodthirsty nature and tyrannical leadership. One of the most appropriate adjectives used to describe Jack would be "manipulative." After Ralph is elected chief, Jack systemically undermines his authority at every turn by encouraging his hunters to neglect their responsibilities. Unlike Ralph, Jack understands what inherently drives the boys and offers them an opportunity to hunt, play, and relax. He also manipulates their fears by contributing to the hysteria surrounding the beast and uses the threat of violence to control his tribe.

Another adjective used to describe Jack is "hostile." Jack demonstrates his hostility towards Ralph by continually arguing with him and physically assaulting Piggy several times. He is naturally antagonistic and creates conflict whenever there is an opportunity. His hostile personality is also on display during his hunting expeditions when he savagely attacks pigs and cuts their throats. As a tyrannical leader, Jack exercises hostility towards his own tribe by randomly torturing members and instructing his followers to hunt Ralph. Overall, Jack's hostile personality and manipulative tactics allow him to successfully rise to prominence and rule his own tribe of savages like a tyrant.

Jack Merridew may be a choir boy, but he's certainly no angel. The antagonist of Lord of the Flies, he's a truly dangerous character whose overweening ambition leads to disorder and bloodshed on the island.

In achieving his ambition to become dictator, Jack shows utter ruthlessness. Contemptuous of rules and democracy, he uses his charisma to gather about him a group of boys looking for thrills and adventure.

Once he's sufficiently strong, Jack then sets about dismantling the rules-based system that Ralph has painstakingly constructed. Jack is so ruthless that he's prepared to destroy anyone who gets in his way, and it isn't long before Ralph is literally running for his life.

As well as being ruthless, Jack is also incredibly cruel, whether it's to pigs or other boys. Jack is a sadist; he actually enjoys inflicting suffering on other creatures.

The littluns do not remotely present any kind of threat to him or his power, and yet Jack still insists on treating them with mental and physical cruelty. Like anyone who revels in cruelty, Jack enjoys picking on the weakest and most vulnerable people in society. More than anything else, this is what makes him such a danger to others. Had the boys not been rescued when they were, there's no telling how much more cruelty he would've inflicted.

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.

What are adjectives that describe Jack Merridew in Lord of the Flies?

To add to what the other editors have said, you might mention that Jack is hedonistic.  He lives in the here and now.  His mantra is "We'll have fun on this island."  He has no use for long-term goals of rescue or shelter.  Instead he enjoys the thrill of the hunt and resents anyone who stands in his way of his savage-like pursuits.  He continues to roll rocks when Ralph has clearly abandoned such pastimes.  When he is not not elected chief in the second election, Jack declares that he doesn't want to play anymore.  Jack cannot get past the fact that the survival on the island is more than fun and games.  He operates solely on the principle of pleasure/pain.  What brings pleasure to him is good; what brings pain to him is bad.

What are adjectives that describe Jack in Lord of the Flies?

Jack Merridew is the red-headed, freckle-faced controlling leader of the choir boys in Lord of the Flies. He has an authoritative air about him, since the choir follows his every command. "Inside the floating cloak, he was tall, thin and bony..." as he peers down at Ralph during their first meeting. He is rude to Piggy and shows unconcerned indifference to his suggestions. Jack later shows his barbaric side when he takes more than an abnormal interest in the hunt. His ruthless actions cause the other boys to fear him rather than admire him. In the end, he becomes nothing more than a murderous thug.