You often hear it claimed that Hitler was an atheist, and this is supposed to demonstrate that atheism leads to moral depravity, or something of that sort. I've heard conflicting claims, but certainly the quotes here seem to indicate he wasn't an atheist.
Not that this shows that theism (or Roman Catholicism, if it can be shown that Hitler was Catholic) is proven to be deviant on the basis of Hitler's actions. Hitler may have thought that God was on his side, but he probably wasn't motivated by his Christianity to kill as he did. He had other motivations. In the same way Stalin was an atheist, but it's not clear that it was his atheism that motivated him to kill as he did. He had a communistic ideology that he thought validated his murderous actions.
On the other hand we do somewhat regularly see religious people do immoral things because of their religious convictions. The obvious example would be 9/11 hijackers. Muslims violence that resulted from the Danish cartoons stems from Islamic prohibition of the creation of graven images. Normal, moral people don't stone adulterers, but Muslims in some countries do because of their religious convictions.
It's easy to pick on Islam in this regard, but it's also easy to find examples of immoral behavior that stems from peoples' Christian religious convictions. Fred Phelps and his church delight in the death of U.S. soldiers because the Bible teaches that God wants to render punishment to nations that turn from him. They believe it is pleasing to God to render this punishment, so if it pleases God it should please us. The Southern Baptist Convention was formed for the very purpose of promoting Southern slavery. It is obvious that religious convictions motivate people to act in immoral ways. It would be interesting to see examples of people that are driven to act immorally on the basis of atheistic convictions.