Between them, they murdered over a hundred million people.
But, as our mentor Robert LeFevre told us, to any extent that a culture has a "public sector", to that extent it is socialistic—potentially capable of the same abuses the more egregious examples of socialism are. Altogether, socialist governments in the 20th century murdered two hundred million of their own people in acts entirely separate from war. Amnesty International calls them "killing machines".
As I say, it's easy to figure out why socialist regimes are always so bloodthirsty. If an individual life possesses little or no value, compared with the continued existence of the group, and if the rights of the individual are few or none, compared with those claimed for the group, the individual quickly comes to be seen as worthless at best, and a threat at worst, to those who pretend to speak or act for the group.