European Chivilary is hardly comparable to the Japanese Warrior Code; Bushido. Chivilary in the Middle Ages was used to bring two comparable warriors on the battle face-to-face, their social standing would have to be equal. By World War II it provided a face of gentlemanship, they were your enemies but they were doing their duty. When the battle is over, the best man won that is all. Those that surrendered were not treated harshly by those that still held the values of Chivilary. There was no disgrace in surrendering in the face of a superior enemy.
Bushido always instilled in the mind to fight to your last breath. By retreating or surrendering, you disgrace your Commander, your Shogun or your Emperor. Disgraceful defeat would mean suicide. This was still the same in World War II.
World War II was Total War. The Allies and Axis all shot men who had surrendered on the battlefield. There were many reasons for that, most of the time for the Axis or Allies on the Western Front it was because they didn't have time to send them to the back or couldn't send them to the back.
On the Eastern Front it was harsh hell and both Soviets and Germans were as bad as one another. I, personally, think that the Soviets were just as bad as the Japanese. The only reason people survived incampment in the Soviet Union was because the Soviets realised they were running short of slave workers.
In the CBI and PTO though, the Japanese acted like savage animals to those they captured. When a man surrenders and you accept his surrender, he is off the battlefield and no longer a combatant. And should be treat like a non-combatant but the Japanese did just as bad to civilians as they did to soldiers. Look at what they did in China.