How is jack presented to the reader?

Jack is presented as a dictatorial leader. He is initially introduced to us as a leader of the choir with malicious undertones,

"Inside the floating cloak he was tall, thin, and bony: and his hair was red beneath the black cap. His face was crumpled and freckled, and ugly without silliness. Out of his face stared two light blue eyes, frustrated now, and turning, or ready to turn, to anger.

As the novel progresses, he becomes more of a tyrant, eventually setting himself up as a paranoid God-King bent on destroying Ralph.