Well DD...it is and it isn't. Marxism is Messianic Humanism hell bent on creating a new uber -being devoted to the collective will and benefit and therefore he is bound to eschew any religion which supercedes the importance of the "new man". He has to be by default an atheist and hence Marxism is by default bound to spread atheism.
One cannot argue that Christianity warts and all has been used as an excuse to kill near the numbers of folks historically that the quest for the "new man" has. Even more startling is that Marxist murdering is only a 100 year old trend where as folks have warred under the guise of Christianity for 2 millenia.
But let's not forget the collectivists of the 20th Century had the dubious benefit of "industrial" killing methods and higher populations of victims not available to those in the past. WWI was amongst Christendom, and managed to rack up a high body count because of the machine gun, the tank, gas, etc.
I don't blame Christianity for the religious wars and crusades and inquisitions - I blame that same collectivist, individual-denying mentality putting group above individual that the Marxists and socialists embraced.