One of the strangest features of the 20th Century was the number of "liberals", "intellectuals", "humanists" or whatever who supported Communist dictators. In any society, at any time, you will find people who make excuses for atrocities committed by their own side. This may be evil but at least it is understandable. The Fellow Travellers justified atrocities committed by the enemies of their own society. This is certainly unusual. Was it unique in human history?
George Orwell tried to explain the behaviour of the Stalinist liberals of the 1930s. I paraphrase, but Orwell wrote that English intellectuals had lost their religion and their patriotism without losing the need for a homeland and something to believe in. Thus they adopted the Soviet Union as a substitute Homeland, Marxist as a substitute Religion and Stalin as their new God.
Actually I don't think that Orwell went far enough. Supporters of genocide and slave labour had not "lost" their religion or anything else. They were consumed with hatred for their Church and Society and they supported a Communist butcher because he shared those hatreds. Thus for example, they did not support Franco because HE was a Catholic and a pro-western dictator. The fact that he killed far fewer people was irrelevant!