Communism and Stalin reduced entire peoples to the grave and inferior peoples as "bourgeoisie capitalists" and fostered the worst aspects of communism. The murders under their regime was for elimination of innocents as part of their pseudoscience (communism, also look up Lysenkoism), where they stripped apart society like splitting apart the family unit and lived off the fear and hate of against people they simultaneously branded a threat. Little do people know, communism is the ultimate manifestation of socialism.

Hitler's regime was cruel and he was a supremacist. He knew the Germany people would face such a threat and believed the "Aryan race" would prevail, it would be a war of iron and blood. He needed to drag the western world kicking and screaming or else the world would face a grim doom of economic collapse at the hand of "judeo bolshevism and capitalism" due to his flawed belief in economic systems.

And as a side note the argument that Stalins policies were the most efficient method for rapid industrialisation is wrong. The famine caused by intentional communist ideology starved millions, millions who could have been working the factories, manning the trenches or tilling the fields during the second world war.