That is not to say that all atheists are evil or capable of such horrible acts. As an atheist, I do not like to share any type of label with Hitler. However, I do not believe that someone who could commit so many horrible acts, and could have caused the deaths of so many innocent people, could have possibly had a religion or spiritual belief system.
He claimed to do everything he did because god told him to. How does that make him any worse than Abraham, who would have killed his own son for god. He was also baptized, gave many public prayers, many speeches about god, and speeches about Jesus being his inspiration for killing all of the Jews. Possibly the most morally consistent Christian in recent history. I do not support his actions in any way.
Hitler was one of the most evil people that there will ever be, but that doesn't automatically mean he was godless. He was an opportunist, saw a group that he could point at while saying "it's not your fault we're in this position, it's them" and exploited it to levels that are beyond comprehension. He was a monster, his religious values didn't play into it.
Adolf Hitler was never a religious man, however he never declared himself an atheist or denied the existence of a god or gods either. In fact, he often referenced divinity or god in speeches or writings, although never in a traditional Christian sense. Hitler most likely believed in some sort of divinity or fate, just not in a traditional Abrahamic sense.
It was not that Hitler did not believe in God, he just picked a group of people, who happen to be a religious society, to focus the peoples anger towards. He would have been more on the idea that God created the perfect man in his image and that anyone outside of that image was not deemed worthy by the Creator.