The atheist friendly Soviet Union and commie China murdered tens of millions.
The pseudo Christian whatever the hell they were Nazis slaughtered 8-12 million, so what is your point? While the numbers were lower (remember they did not have mechanization), the Inquisition and witch trials did a good job of slaughtering a bunch of folks indiscrimately. You also had the pograms against the Jews in Spain, Russia, etc.
A website describing the various Christian atrocities:
http://www.truthbeknown.com/victims.htm
Some more facts - Leopold II (a Catholic) slaughtered 8,00,000 in the Congo
From 10 to 100 million Native Americans (actual number unknown from Christian Europeans and their diseases)
50 million? in Africa from colonization and the slave trade
How many in Asia from colonization as well?