From the beginning of the novel, Jack is rude. In chapter one, upon first meeting Ralph and Piggy, he calls Piggy "fatty." He has no sensitivity for Piggy's feelings:
'You're talking too much,' said Jack Merridew. 'Shut up Fatty.'
Clearly, Jack has no manners. He gets a laugh from the other boys at Piggy's expense.
In chapter one, as the boys are electing a chief, Jack is arrogant:
'I ought to be chief,' said Jack with simple arrogance.
From the very beginning, Jack is obnoxious. Clearly, he feels superior to the others. The reader automatically dislikes Jack based on his characterization. He is bossy and forward in his actions and dialogue.
By chapter four, Jack is a mighty hunter. He declares himself a great hunter. He screams out, "'Kill the pig. Cut her throat. Spill her blood.'" This is his reaction to the pig he has just killed. He has no mercy. He is cold hearted in his hunting frenzy. It seems to not affect him that he has just killed a living creature.
As the story progresses, Jack becomes more savage in his actions. He leads the boys in a hunting dance that ends with the murder of Simon. Jack seems so hard hearted. He becomes more savage each time he kills a living creature. In chapter nine, Jack has his boys screaming and chanting, "'Kill the beast. Cut his throat. Spill his blood.'" Then Simon comes crawling out of the woods and they begin to kill Simon with their bare hands and teeth.
With no remorse, Jack continues to control his boys. He is power hungry. In chapter eleven, Jack and Ralph argue about who the leader is. When Piggy falls to his death because of Roger, Jack indicates that Ralph will get the same treatment:
Jack screams that that’s what Ralph will get. The conch is gone. He is chief now. He hurls his spear at Ralph, grazing his side. The others, including Roger, hurl their spears as well. Ralph turns and flees...
Jack becomes more antagonistic as the story unfolds. By the end of the story, he is in total control over the boys. Ralph is the only one who has not succumbed to Jack's tyranny, and he is fleeing for his life.
Jack is a reprehensible character, and his abusive language only worsens as the story unfolds.