The first quote that I have in mind clearly shows Jack's initial desire for rules, order, and civility.
"We've got to have rules and obey them. After all, we're not savages. We're English, and the English are best at everything."
This quote comes from chapter 2, and it clearly shows that Jack is on board with things like having a leader, establishing rules, and people following rules. Unfortunately for everybody involved, Jack does not enjoy following the rules that have been set forth by anybody other than him.
I have two quotes in mind for showing Jack struggling with staying civil. The first one is from chapter 3.
[Jack] tried to convey the compulsion to track down and kill that was swallowing him up.
"I went on. I thought, by myself—"
The madness came into his eyes again.
"I thought I might kill."
This shows that Jack's desire for savagery and killing is there. It shows that it is building, but Jack is currently keeping it at bay. In the next chapter, readers get a view that shows Jack losing control and letting some of his new savagery be seen by others.
[Jack] began to dance and his laughter became a bloodthirsty snarling.
The "bloodthirsty snarling" is something that readers would associate with a savage beast. He is no longer an innocent choir boy.
The final quote that shows Jack having fully embraced his inner savage is from chapter 8. It is the part where Jack mutilates a pig during his kill.
The spear moved forward inch by inch and the terrified squealing became a high-pitched scream. Then Jack found the throat and the hot blood spouted over his hands. The sow collapsed under them […].
At last the immediacy of the kill subsided. The boys drew back, and Jack stood up, holding out his hands.
He giggled and flecked them while the boys laughed at his reeking palms. Then Jack grabbed Maurice and rubbed the stuff over his cheeks . . .
“Right up her ass!”
At this point, Jack's hunting is no longer about acquiring enough food for everybody. Jack hunts and kills for the pleasure of stalking his prey and killing it. However, Jack enjoys more than just killing his prey. He enjoys torturing his prey during the kill. There is nothing civil about Jack's actions at this point.
Jack's transformation from proper English schoolboy to ruthless, savage killer is one of the most shocking in the Lord of the Flies. Like the naval officer at the end of the novel, the reader, too, is left wondering how come a group of nice, English boys could so quickly deteriorate into a wild pack of savages. The answer, of course, is Jack. His challenge of authority and own personal descent into violence sets an extremely negative example for some of the other, more impressionable boys.
The first moment in Jack's regression into a savage occurs the first time the boys experiment with making masks. Jack "looked in astonishment, no longer at himself but an awesome stranger" (63). The mask liberates Jack from his inhibitions, giving him the boldness to act, freeing him from "shame and self-consciousness" (64). Only moments later, Jack makes his first kill.
"There were lashings of blood," said Jack, laughing and shuddering, "you should have seen it!"
The thrill of the kill is upon Jack, making him feel powerful and respected for his ability in the tribe among the other boys. Jack has come to see violence as a tool, helping him to gain power in the tribe. This sentiment is never more evident than immediately after Piggy's death. Jack uses the horrifying moment to solidify his power over the other boys:
"Suddenly Jack bounded out from the tribe and began screaming wildly.
'See? See? That's what you'll get! I meant that! There isn't a tribe for you anymore! The conch is gone--'
He ran forward, stooping.
'I'm chief!' (181)
Any aspect or claim to civilization on the island died with Piggy and the destruction of the conch. By now, Jack has completely given himself over to savagery, first by demanding Ralph's exile and then later by planning his death. Ralph can only guess what fate awaits him at Jack's hands:
'What could they do? Beat him? So what? Kill him? A stick sharpened at both ends" (198).
The stick sharpened at both ends suggests that Jack means to kill Ralph and then mount his head on the stick, as he did the sow's head. Jack has truly become a savage; by planning to treat Ralph as the hunters did the sow, Jack has completely lost all value for human life.