I’d been thinking of that same quote, except that it was Trotsky who said it. Of the two of them, it’s hard to say who was the more cruel, stupid, and bloody. Of course, Stalin and Mao topped them both.
You are entirely correct about the incredibly bloodthirsty nature of both Stalin and Mao. (Just how Mao might ever have become a cult hero, among some on the left, is utterly beyond me.)
Perhaps the bloodiest tyrant of the twentieth century--not in absolute numbers, but in terms of murders as a percentage of the total population--was Pol Pot, whose Khmer Rouge had up to three million people murdered, out of a population of only about eight million in Cambodia at the time (or "Kampuchea," as it was briefly renamed).