Marxist-Leninists claimed to have a scientific doctrine, a philosophy based on human reason alone devoid of faith or mystical revelation. Stalin and Mao claimed to be followers of Marxist-Leninism, and they claimed (and apparently sincerely believed) that the tens of millions of murders they committed were justified by the philosophy they held. It is therefore perfectly reasonable to say that Stalin and Mao killed millions of people in the name of science and reason.