Chapter 4 is a great place to start. During this chapter, Jack undergoes a transformation particularly after the children paint their faces following his example. This narration portrays his transformation:
He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and 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.
This passage demonstrates his awe at his own developing character. His "laughing excitedly" reminds me of an evil villan about to terrorize a town. The mask threatens his friends demonstrating his dictatorship. He sheds his inhibitions and no longer fears the opinions of others, but from this point forward he certainly envokes their fear.
Jack's moment of complete selfishness occurs when he leaves the group because they won't elect him chief over Ralph:
“I’m not going to play any longer. Not with you.”
Most of the boys were looking down now, at the grass or their feet. Jack cleared his throat again.
“I’m not going to be a part of Ralph’s lot—” He looked along the right-hand logs, numbering the hunters that had been a choir.
“I’m going off by myself. He can catch his own pigs. Anyone who wants to hunt when I do can come too.”
This entire passage demonstrates that Jack cares only about himself and his need for authority over others. Because they refuse to choose him as leader he just quits the society. This happened in chapter 7.