Does Ralph understand why he must be killed? Explain.
Early in the final chapter, Ralph, in hiding, watches the boys as they feast. He notes that their savagery has increased tremendously. There is no mistaking their similarity to uncivilized savages. As he thinks about all that has happened, he refuses, at first, to believe that the boys could have purposely become killers. Then he encounters the skull of the sow that Jack killed and impaled on the stick. It appears to be grinning at him regards Ralph, "...like one who knows all the answers and won't tell." Ralph realizes, then, what Simon and Piggy knew - that the real beast, the real source of evil, was inside of each one of them. He also slowly comes to realize that just as Simon and Piggy died for gaining this insight, he, too, is likely to die. When he encounters Sam and Eric and they tell him that Jack and the tribe plan to hunt him like a pig and that Roger has sharpened a stick at both ends, Ralph feels even more certain that his fate is to follow Simon and Piggy. Even though he never articulates the idea as much as Simon or Piggy, Ralph has come to realize that understanding the source of evilness comes with a price and that price is death. At the end, when he is rescued instead of killed, he weeps for, among other reasons, the loss of innocense. His realization of the source of evil robs him of ever being innocent again.
In a word, no. Samneric give him meat and tell him the Chief's plans. And there's no reason for those plans:
“They hate you, Ralph. They’re going to do you.”
“They’re going to hunt you tomorrow.”
“I dunno. And Ralph, Jack, the chief, says it’ll be dangerous—”
“—and we’ve got to be careful and throw our spears like at a pig.”
“We’re going to spread out in a line across the island—”
“—we’re going forward from this end—”
“—until we find you.”
“We’ve got to give signals like this.”
Eric raised his head and achieved a faint ululation by beating on his open mouth. Then he glanced behind him nervously.
“—only louder, of course.”
“But I’ve done nothing,” whispered Ralph, urgently. “I only wanted to keep up a fire!”
He paused for a moment, thinking miserably of the morrow. A matter of overwhelming importance occurred to him.
“What are you—?”
He could not bring himself to be specific at first; but then fear and loneliness goaded him.
“When they find me, what are they going to do?”
The thing is that there is a reason: this is what leaders like Jack do to people who don't agree with them and don't submit to their leadership. They eliminate the competition. And that is what Jack is going to do.