Of Tu Quoque and Atheism as a Motivation for Mass Murder
The church has a long tradition of killing free-thinkers specifically for religious reasons. Pointing the finger at others will not erase this fact. But let's consider the claim that atheists "did it too." Let's consider the popular example, Stalin. Is the religious apologist's tu quoque that atheist Stalin was motivated to kill by his atheism valid? Was Stalin driven by atheism to kill not just Christians but to conduct the wholesale slaughter of human beings generally?
Stalin was a mass murderer who killed for political reasons. Stalin killed millions of people for Stalin. The people he killed included both Christians and atheists. It is clear that he killed Christians at least in the beginning as part of the party agenda. Atheism was a tenet of Marxist ideology, but it would be hard to imagine how Stalin would have been driven to kill by atheism alone. The salient question is how can one be driven by atheism, a lack of belief? Was Stalin motivated by atheism or was he motivated by Marxist ideology in addition to a desire to destroy all authority that was not himself?
Lack of belief cannot motivate one to do anything. The lack of something cannot logically do anything. But ideologies can. If you make atheism a tenet of an ideology like Marxism, then the ideology can in certain circumstances drive people to seek the destruction of religion, though it does not follow that this is necessarily always the case or that such an ideology would motivate the indiscriminate slaughter of human beings generally. It is well-established, however, that putting power in the hands of paranoid sociopaths like Stalin, Caligula or Nero invariably does lead to problems.
At most we can say that Stalin was motivated to kill by his desire for power, Marxism, and his own psychopathy. Though there is some doubt as to whether Stalin even cared about Marxism. It may simply be that he saw an opportunity and seized it, taking advantage of the revolution to maneuver himself into a position of power. It seems Lenin began to have doubts about Stalin's motives towards the end of his life. Lenin's testament concluded with a recommendation that Stalin be removed from his position as secretary-general of the party.
On a side note, populations have grown exponentially over the past two centuries. There were more people in the 20th century than had existed in the previous two millennia, and there existed for the first time technology to make extermination of human beings possible on a scale never before imagined. If there had been a 20th-century-sized population in medieval Europe, and if the church had had access to 20th century weapons of mass destruction, how many heretics, pagans, and atheists might the church have killed?