The death toll of communism is debatable because a lot of deaths in the Soviet Union and China were due to famine, and people can argue how much of it was due t

The death toll of communism is debatable because a lot of deaths in the Soviet Union and China were due to famine, and people can argue how much of it was due to the inefficiency of collective farming, or deliberate starving of people the government didn't like, or just bad luck. 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.