Five Decembers Q1: When is killing justified?
I think this novel invites us to question when killing (and other violence) is justified. McGrady kills four people in the book: the accomplice at the murder barn, the Kenpeitai boy, Beamer during interrogation, and John Smith. Other killings were the two prisoners in the transport ship, beaten and executed for talking; John Smith's three victims, tortured to death; and Kam's family, killed during the capture of Hong Kong. Bracketing those were Fred Ball's beatings of prisoners (and McGrady's beating of Ball in retaliation), and the death of 100,000 civilians in the firebombing of Tokyo.
Which of those killings were justified? Which would be classed as "legal" in a court? Which were avoidable? What were the costs of those killings, compared to the costs of not killing, or taking other steps to avoid the deaths?

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Comments
For example, the deaths in Tokyo from the bombing, or similarly those on Hawaii during the Pearl Harbor attack, or those during and after the Japanese capture of Singapore etc - all those are matters of history, and one couldn't really write about those places and times without touching on them. The others you mention, carried out by McGrady, Ball and Smith, are literary inventions in the book. I didn't get the sense that the deaths were gratuitous, although Ball's treatment of prisoners seems to modern thinking quite appalling (I suppose from McGrady's reaction they were out of line at the time, but I don't know know enough to say for sure).
I agree that the descriptions of the deaths don't overly dwell on the gory details. There's enough there that you can imagine the scene, but not so much as to revolt the reader (or at least, me).
But yes, when is killing morally justified? What killings, portrayed in the book, are justified?
Quite a different level of detail from The Orenda, for example