I don’t think of the Japanese as being an unusually violent people. However, I’ve read that the Japanese army had training programs which encouraged cruelty and atrocities by their troops. I also suspect that alcohol was a factor in atrocities by the Japanese and Russians, just as drugs are used by African warlord to encourage frenzies of cruelty by their forces.
I’ve read that the japs were taught that surrender was not an option, that it was dishonorable, and thus any captives from the other side that they took were dishonorable as well. Then throw in the inherent Japanese racism, and voila, atrocity.