Also regarding 3, I wonder if, comparatively, the Soviet Union forcefully moved a lot more people (even percentage-wise) through its network of concentration camps. African slaves going over the Atlantic were what.. 2-3M deaths over 300 years, whereas the USSR easily disposed of 10-30M between 1917 and the 50s and later in its various GULAGs.
Much better organized, more ruthless, exterminating its own lawful citizens, thanks to the wonders of ideology. All for the greater good of society and the working class. No ethnic difference necessary to justify unprecedented genocide. Neighbors reporting neighbors to the authorities with almost total certainty of execution, for many decades.