During World War I, Imperial Japan more or less adhered to the same standards regarding the treatment of enemy prisoners as the other nations involved. They were, if not exactly nice to them, at least humane, and in the end allowed them to live.
Come World War II, however, all of those standards were flushed down the proverbial toilet. Being in a Japanese prisoner-of-war camp during World War II was, to put it bluntly, a death sentence. So my question is, what happened? What caused Japan to pivot so dramatically on its treatment of enemy prisoners over the space of 20 years?
Please see here by /u/amp1212. In summary, before the 1930s the Japanese politicians cared about international opinions towards their conducts. Since the 1930s, they didn't.