It's hard to say who was the worst - The Japenese were excedingly cruel both to combattants and to civilians. It is reported that 250,00 Chinese were excuted for knowledge of/help to the Dolittle raid and that was the tip of it.
The Germans tried to wipe out an entire ethnic group and tried to wipe out the commies.
Russia would send political prisioners in large groups (it's said this total could millions) without wepons ahead of their armies into combat to wast the Germans amo. The totals of people sacrificed/worked to death is said to be over 20 million.
What's the definition of bad?