4 ideas
In January 1945, Ōoka was a feverish, abandoned straggler on Mindoro. A young American soldier walked into his line of fire, and Ōoka did not shoot. The episode tests any theory of why soldiers refrain from killing, because the person who held the rifle could not settle his own reason.
Ōoka takes his own non-shooting apart and rejects the noble readings of it, such as humanism or a pacifist conscience. He sets out the other candidates beside them: paternal feeling toward a youth, physical exhaustion, a reluctance to trigger his own death, and the plain accident that the American turned away. The mechanism is that a morally admired act can come from several mundane causes at once, so the actor's later explanation tells you more about his self-image than about why he acted.
Once fed and safe in the Leyte camp, Japanese prisoners rebuilt a smaller version of the society they came from. They formed pecking orders, traded favours, sank into petty rivalries and flattered their captors. Ōoka reads the camp as a preview of occupied Japan. When a group loses both its cause and its enemy, it does not become free; it reproduces its old habits of status-seeking in a degraded form.
On Leyte, supply collapsed and units disintegrated. Unit loyalty and the military code that treated capture as worse than death gave way once bodily need took over. Discipline that rests on shared provisioning ends when the provisioning ends, well before the army is formally beaten.