The Japanese are an interesting case, an extreme case, and perhaps a unique case. It's very hard to reconcile their more or less gallant conduct prior to and during the Russo-Japanese war with the way they acted from about 1932 onwards. It's possible that earlier they wished to seem more like the west, then, hence tried to be more like the west. It's also possible that the Great Depression changed them. It also seems to me possible that the coming death of Bushido, which Nitobe Inazo predicted, caused an extreme reaction to the point of the psychotic. Or it could be any combination of those or other factors.
I wouldn't say that it's that difficult to reconcile. The IJA patterned itself on French and German models, and during both the First Sino-Japanese War, the Russo-Japanese War, and the Boxer Rebellion sought to accommodate itself to international norms of conduct, even in conflicts when Western observers were not present (First Sino-Japanese War, Korean occupation). IJA leadership specifically set this as a goal and made it a priority for troops in the field. This led to IJA behavior towards POWs and civilians that was markedly better than most Western armies (the contrast between Japanese and German behavior in the Boxer Rebellion was most marked).

A combination of the rejection of international norms due to the international condemnation of Japan's invasion of China in 1932 and the grinding brutalization of an endless guerrilla conflict in China led directly to the decline in IJA behavior. The IJA high command had undergone a generational shift between the RJ War to a much more aggressive, fascistic, and ultranationalist leadership. The difference between a leadership which subordinated itself to civilian authority and sought to enforce international norms of conduct and one that dictated to civilians and made a fetish of rejecting international opinion cannot be underestimated.