Stalin killed those he saw as opposition. He didnt just indiscriminately kill for the sake of it.Maybe they have a stop button and don't consider it too bad yet. No one knows how many Mao murdered for sure. We have a better idea how many Stalin murdered because there are some sketchy records here and there. I think that they take the census records in Russia and from the birth records they know how many were added so subtracting the newborns and using whatever death records there may be, they can calculate the shrinking population. American Stephen Kotkin is the world expert on Stalin and this is what he said in a lecture once on YouTube. Also, Solzhenitsyn had good information in the Gulag Archipelago books.