intense feelings of suffering; acute mental or physical pain
"Agony" is from the Greek "agon" which means "struggle" and can be seen in the words "protagonist" and "antagonist." Usually, a protagonist is agonized because of a struggle with a specific antagonist. Here, Jonas is struggling with the pain of a memory of a broken leg. The antagonists that cause Jonas agony are not identifiable humans he can fight; rather, they are the world's memories of the past and his present community's attitudes and approaches towards life.
In his agony he perceived the word “fire” and felt flames licking at the torn bone and flesh.
"Brutal" also means "resembling a brute or beast; showing lack of human sensibility"--this definition is the opposite of what the brutal pain does to Jonas: while it may temporarily reduce him to a screaming pile of flesh and bone, it also makes him more aware of what being human means and feels like. The Giver explains, “It gives us wisdom.”
"Torturous" and "excruciating" are synonymous adjectives with different roots: "torturous" comes from the Latin "torquere" which means "to twist" (and can also be seen in "contorted"); "excruciating" comes from the Latin "cruciare" and "crux" which mean "to crucify on a cross"--this could suggest that the Receiver is a Christ-like figure, since both were chosen to take on the painful burdens of the larger community.
"Why?" Jonas asked him after he had received a torturous memory in which he had been neglected and unfed; the hunger had caused excruciating spasms in his empty, distended stomach.
"But then everyone would be burdened and pained. They don’t want that. And that’s the real reason The Receiver is so vital to them, and so honored. They selected me—and you—to lift that burden from themselves.”
"Grotesque" means "distorted and unnatural in shape or size" (often to a monstrous degree)--this is a fitting adjective for the colors of carnage, especially to a boy who is unused to seeing color and images of war.
The colors of the carnage were grotesquely bright: the crimson wetness on the rough and dusty fabric, the ripped shreds of grass, startlingly green, in the boy’s yellow hair.
Finally, when the container was open, he extended his arm slowly across the blood-soaked earth, inch by inch, and held it to the lips of the boy. Water trickled into the imploring mouth and down the grimy chin.
Jonas is shuddering because he is struggling not to cry. Having experienced war through a memory, he is distraught at the sight of children giggling while they pretend to kill and die. Although crying is often seen as weakness, Jonas shows courage when he walks into the center of the field, stands alone against the players, and displays an intense emotion that the others don't know how to respond to.
Then they were all silent, standing awkwardly, and the only sound was the sound of Jonas’s shuddering breaths.
“Me,” Jonas said in a dejected voice. He was not looking forward to the end of the training, when he would become the new Receiver. It was clear to him what a terribly difficult and lonely life it was, despite the honor.