Almost 50 years prior to WWII the Japanese military in the Russo-Japanese War & WWI were an honorable foe. Their treatment of Russian POWs was exemplary as was the few German & Austro-Hungarian POWs they had from WWI. There was international praise on the Japanese military’s behavior. I’ve never read a convincing historical study as to why this changed. Most I’ve read blame it on the racist attitudes of the West particularly at the post-WWI disarmament conferences where Japan was not treated as an equal ally. Maybe so, it still seems to me a too easy of an explanation. I find it inadequate.
Japan, like Germany, was overtaken by the then new crackpot Eugenics theories. We can than Behavioral Pseudoscience for this. Even today crackpots and various psychopaths believe than your genetics determine everything about your thinking and behavior as well as physical attributes. When this idiot theory gains traction the only “solution” to problems is to murder the “unfit”. Declare a race or even a political faction a mental disease and then concentration camps become a public service.