Worse, actually.
They literally went went all Hannibal Lector on their POWs. There were multiple testimonies and even a few convictions, though these were usually charged as murder with "prevention of honorable burial" because this was so unexpected that the military tribunes didn't even
have cannibalism on the books as a crime - they had to make do with what they had.
And then there was Unit 731, where they were working on actual biological weapons by using Chinese people as their test subjects. They were literally planning on releasing deadly diseases for which only their own soldiers would be immunized against.
In fact, they already were using biological weapons against the Chinese by doing things like spreading disease-carrying fleas, contaminating water sources with water-born diseases, and even giving out food contaminated with anthrax.
You can find all of that and more here:
https://en.wikipedia.org/wiki/Japanese_war_crimes