Can't speak for Islam, but the number of souls killed by the various agents of Christendom is a FRACTION of what Stalin himself accomplished in a single decade.
It's likely that fewer people in Western Europe were killed or died (including by natural causes) in total numbers throughout the entirety of the medieval period (from the reign of Charlemagne to the Protestant Reformation in 1517)than were exterminated by Communists in the 20th-century. If you add modern abortions in secular humanist countries as examples of genocide, then that is certainly the case.