The Problem of Miscommunication
Christians argue how that Christianity helped abolish slavery. Big deal, even if this is true. Who are you trying to kid here? If God had condemned slavery from the very beginning there would be nothing to reform, no beatings, no killings, no institutional slavery justified from the Bible. If God had repeatedly said, "Thou shalt not buy beat or own slaves," and never sent any vibes the other way, then Christians could never justify it as an institution.
There are so many other examples. Did you know that 8 million Christians killed each other during the French Wars of Religion and during the Thirty Years War over the real presence of Christ in the Eucharist and who was the legitimate authority to administer it? Again, 8 million Christians. All Jesus had to do was to say he was speaking "metaphorically" about his body and blood, if that's what he meant. And all Jesus had to do was to clear up whether Peter was the rock or his confession was the rock in Matt 16.
To say these Christians (both Catholics and Protestants) should've known better is sheer ignorance, for they still disagree about this today. Whenever there is miscommunication both parties are to blame, especially if there is an omniscient God who could've known in advance how his followers would misinterpret what he said. Do you understand this? All attempts at answering this particular problem utterly fail to take it seriously.
For Christians to claim atheism was the cause of many deaths in modern wars misses the point for three reasons: 1) Atheism per se was not the cause of the killings; 2) It's a red herring, since whether or not this was the case with atheism does nothing to solve the Christian Problem of Miscommunication; 3) If the Christians in that era had modern weapons of war, including nuclear weapons, then we would've seen many more deaths, possibly genocide.