In chapter four, Jack is thinking about the killing of the pig. He is absorbed in the moment after the kill. He is thinking about the thrill of the kill:
His mind was crowded with memories; memories of the knowledge that had come to them when they closed in on the struggling pig, knowledge that they had outwitted a living thing, imposed their will upon it, taken away its life like a long satisfying drink.
In this passage, Jack is in command. He has imposed his will on another living creature. He is ecstatic. He has no sympathy for the animal. He is prideful in his killing of the pig,
Jack feels all powerful in that he has outwitted the pig. He has proved himself more powerful than the living creature. He is in command.
The beast within Jack is evident when he compares the taking away of the pig's life to a long, satisfying drink. There is nothing satisfying about killing another living creature, yet, Jack is exhilarated. He is fulfilled:
'We needed meat.'
Jack stood up as he said this, the bloodied knife in his hand...There was the brilliant world of hunting, tactics, fierce exhilaration, skill...
Jack is a hunter. He is aggressive. He is a killer. He has the beast within him.
For the "beast within" quote, I like this one 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."
I like the quote a lot because it shows Jack still struggling with the beast within. He hasn't fully embraced it yet. The quote is showing readers a hint of Jack's true nature that society has likely kept in check all this time.
For a quote that encompasses the "lack of sympathy," "power," and "torture" part of the question, I would use the pig killing scene from chapter 8.
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!”
It is clear from this section that the kill for Jack is no longer about food and keeping people fed. He takes pleasure in the kill, but there's more than that. Jack isn't satisfied with a quick, clean kill. He has to torture the kill with inappropriate cuts.
For a quote that encompasses the "commanding," "pride," and "jealousy" part of the question, I would use the quote from chapter 1 when Jack petitions to be the chief.
"I ought to be chief," said Jack with simple arrogance, "because I'm chapter chorister and head boy. I can sing C sharp."
The quote shows that he wants to be in command. His tone of arrogance shows his prideful nature. The quote hints at his jealously as well because he can't believe that the other boys would even consider Ralph to be chief.