In WW1 Japan treated POWs quite leniently as evident in places such as BandÅ prisoner-of-war camp, where conditions were so good quite a few chose to stay in Japan after the war. Fast forward to WW2, then you have horrible things like the Bataan Death March, or POWs used as slave labour to build the Burma Railway, to name a few examples.
What changed?
More input is always lovely; in the meanwhile, your question actually popped up two weeks ago, with input from u/amp1212 (who also has more on the topic in another thread linked there) and u/SteakSundae.