From hemingway a farewell to arms in the excerpt, what does the diction of words used to express an idea, show about the narrator

I bet your question is connected to these lines (which are common in use for analysis) : “It could not be worse,” Passini said respectfully. “There is nothing worse than war.”
“Defeat isworse.”
“I do not believe it,” Passini said still respectfully.
“What is defeat? You go home.”
“They come after you. They take your home. They take your sisters.”  If so, the answer is that the author uses informal diction in  the passage to create  a deeper meaning and to maintain the realism of the dialogue.