The Germans (and Swiss...) for the most part imprisoned / shot PoWs.
The Japanese used PoWs for chemical and biological weapon testing and vivisection, and they would find the most depraved ways to draw out mass torture of PoWs.
For example, they would do things like put US soldiers into baskets so small PoWs could not move at all inside them, load them into unventilated railroad boxcars with no water, then load them into ships, sail out to sea, and drop them overboard into shark-infested waters.
Then there's the Nanjing Massacre where in a matter of days Japanese soldiers slaughtered something like 300,000 Chinese civilians at a rate of around 7,000 civilians a day. That was just one of the more than a dozen massacres. It's widely believed that Japan slaughtered more civilians than the Germans.
The sheer level of depravity is astounding. And what's especially disturbing are the lengths conservatives in the Japanese government have gone to over the last several decades to rewrite history and erase these atrocities, while pushing harder and harder against the restrictions on their armed forces.