I am genuinely curious about where you get the figure of 100 million deaths from. I also believe that all the societes that had slaves were capitalist ones.
But we can both agree that communism leads to poverty although the poorest countries in the world are failed states rather than communist: https://www.worldatlas.com/articles/the-poorest-countries-in...
As for "all societies that had slaves were capitalist" -- it is unclear what you mean by that. Certainly the US was capitalist and had slavery, but many (indeed most) pre-capitalist societies had slavery.
The highest death tolls that have been documented in communist states occurred in the Soviet Union under Joseph Stalin, in the People's Republic of China under Mao Zedong, and in Cambodia under the Khmer Rouge. The estimates of the number of non-combatants killed by these three regimes alone range from a low of 21 million to a high of 70 million.
German POWs were kept as slaves until, IDK, the 80's?