I remind them that the entity that killed all the millions on earth, save 8, with the flood, was a Christian.
It depends on who they bring up. As many others have already said, Hitler was Catholic and stated many times that he considered his actions as driven by his religious beliefs, including the eradication of atheism and secular schools. Stalin, Mao, Pol Pot and the other usual suspects are almost always communist dictators who had their own dogma (either the cult of self like Stalin or Communism) that drove their actions.
The important point to drive home that it is what drove their actions that is important, not things that were not behind their actions. For example, both Stalin and Hitler has mustaches but no one would claim that therefore mustaches were behind their actions.
I disagree with Steven Weinberg's quote that Sassan mentioned, if only because it is too specific. For good men to do evil you do not need religion specifically, you need dogma and unquestioning faith. While religion may be the main source of these things, there are others. Dogmatic belief in a nation, economic system, person, etc can all cause the same effect.
I like when Dawkins said "Hitler was also a vegetarian". It is a really funny presentation he gives. If you can find it on youtube its worth watching. Dawkins also points out that Hitler wasn't an atheist, as many of these fine minds on TA already have, he will still argue about it.
There's no shortage of evil Xtians in history either. Just off the top of my head Hitler, Torquemada & Vlad Dracul come to mind. Plus you have the KKK, all the Catholic (& other) pedophiles and those who enable them. Nearly any evil person in western civilization for the past 2000 yrs was almost certainly Xtian.
Doesn't matter if we're atheist, Christian, Jew.... Reality is unchanged by our "belief". The Universe, by all evidence, does not have gods that look after (or bully!) humanity. Atheists simply acknowledge the fact. Trying to point to failed individuals who are Atheists or Christians has not the slightest influence on the validity of their respective claims. The evidence speeks to that.
Atheism has nothing to do with the behavior or ethics of people. It simply means that religion is unacceptable as a truth. There are far more religious people who one could classify as evil if you wanted to use that Manichean term, the use of which is ambiguous and I prefer dysfunctional. Evil is a religious word.
Evil is not a scientific term.
Hitler also, to reinforce Dawkin's observation, also outlawed the practice of pate fois gras, from Strasbourg, the inhumane force-feeding of geese to fatten them up for human consumption.
So evil becomes a relativistic judgement rather than a scientific observable phenomenon.
What do you say when they are talking about Hitler? Some people truly believe that Hitler was an atheist.