The Japanese were vicious to everyone...including their own soldiers. Military training in Japan was full of relentless and savage brutality on the part of officers (many of them) toward the enlisted men. This had no small effect in causing those troops to be brutal to prisoners and populations in occupied areas.

It was a shameful misuse of power, encouraged from the top down. I have read several books by Japanese Navy and Air Force veterans. Without exception, they express their disgust for the cruel treatment meted out to young soldiers, sailors, and pilots by their sadistic officers. The better men rose above that sort of thing and did not fall into the trap of imitating it. The weaker and less independent ones imitated it. The same sort of thing has happened recently in Iraq, with poorly educated and poorly led young American soldiers brutalizing Iraqi prisoners in a fiendish manner...but it would be hard to find any example of it that exceeds the terrible Japanese behaviour in World War II.