Contrast Jack's behaviour at the end of Lord of the Flies with that of Ralph's. Why does each one act in such a manner?

By the end of the novel, Jack's behavior is reprehensible.
He has only become more antagonistic at the story's closure. When Roger causes Piggy to
fall to his death, Jack declares himself chief and screams at Ralph that he will get the
very same treatment:

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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...

While Jack has
become more and more obnoxious, Ralph has become more humble. He is literally running to
save his life from Jack's tyranny. Ralph only desires democracy and civilization. Jack
desires total control over the boys.

Jack is a wild hunter.
He has become savagery. He thrills over the kill. He works the boys up to a murderous
frenzy in the hunter's dance. At first, it is playful behavior, but soon the boys begin
hurting Robert who is playing the part of the wild
boar:

Jack
grabs him by the hair and brandishes his knife. ‘Kill the pig! Cut his throat! Kill the
pig! Bash him in!’ Ralph too was fighting to get near, to get a handful of that brown,
vulnerable flesh. The desire to 'squeeze and hurt was
over-mastering.'

Even Ralph
gets caught up temporarily in the murderous frenzy of the dance, but, by the end of the
story, Ralph has lost all hope of surviving. He realizes he will be Jack's next victim.
Fortunately, Ralph runs into the naval officer. All Ralph can do is sob over the
terrible things that he and Jack have done.