Omar you raise an interesting point. I would agree that the Japanese were far more savage to the allies, both military & civilian then the Germans were to the Western Allies.
But the Germans were definitely the most savage when it came to fighting on the Eastern Front. Japanese military observers were shocked at the widespread brutality of the Nazis as they pushed across Russia in 1941. One Japanese military attache asked their German counterpart "there won't be anything left for you to exploit" and the German apparently replied "that's the intention".
I suspect the level of savagery in these cases is dictated by racism. The Japanese had racist views of Americans, Chinese, Australians, British and Filipinos. The Nazis had deeply racist views of Russian Slavs.
The war between Hitler & Stalin was the most savage in human history. The fact was that Nazi Germany AND Soviet Russia were among the most evil & odious nations in human history. That their war would be the nastiest civilization had ever known was almost predictable.
If you want to check out a great book on the Soviet/Nazi war from the German POV read "The Forgotten Soldier". This guy lived through fighting on the Eastern Front and he's no apologist for the Nazis. A stunning read.
Amazon.com: The Forgotten Soldier: 8601400965771: Sajer, Guy: Books
www.amazon.com