A German bishop has argued that atheism causes and has caused all manner of suffering/genocide/mass murder. Which is to say, he thinks that atheism is as evil as the God that he worships.
The story can be found here: German bishop: Atheism responsible for Nazis and mass murder
The story quotes several atheist organizations that attempt to correct the factual errors of what Bishop Mixa has said. By continually arguing the facts of the matter, we atheists are allowing our opponents frame the debate. When an atheist organization claims that the majority of Nazis considered themselves Christian, modern day Christians are likely to think something like, 'well, they obviously weren't very good Christians, but if they were atheists instead just think how much worse it could have been!'
I think a more fruitful approach for an atheist organization and atheists in general would be to respond with something like the following:
Hypothetical Atheist Organization Response:
The bishop can argue that atheism leads to behavior as terrible as that of the God that he worships. Of course, as atheists we have no issue condemning the mass murder that God commanded as well as the mass murder that Hitler commanded.
A major defining difference between our two organizations: the church is the only one seeking praise for the actions of a mass murderer.
We welcome any moral progress that the bishop wishes to demonstrate by condemning the acts of God that he would otherwise so rightly criticize: mass murder and genocide.
For reference I have included two very evil verses that demonstrate the actions and commands that the God of the Old Testament that the bishop has praised.
16But as for the towns of these peoples that the Lord your God is giving you as an inheritance, you must not let anything that breathes remain alive. 17You shall annihilate them—the Hittites and the Amorites, the Canaanites and the Perizzites, the Hivites and the Jebusites—just as the Lord your God has commanded, 18so that they may not teach you to do all the abhorrent things that they do for their gods, and you thus sin against the Lord your God.
38Then Joshua, with all Israel, turned back to Debir and assaulted it, 39and he took it with its king and all its towns; they struck them with the edge of the sword, and utterly destroyed every person in it; he left no one remaining; just as he had done to Hebron, and, as he had done to Libnah and its king, so he did to Debir and its king. 40So Joshua defeated the whole land, the hill country and the Negeb and the lowland and the slopes, and all their kings; he left no one remaining, but utterly destroyed all that breathed, as the Lord God of Israel commanded.