I agree, the details of Hitlers views are hard to pin down. But I think that lumping him into a list of "atheist mass murderers" is extremely misleading. Aside from a few anecdotal accounts of skepticism, we have little reason to believe Hitler was not religious and many reasons (including his own statements) to believe that he was.
These arguments are also beside the point, as there is a key difference between the three men listed above, and the religious zealots they are being compared to. The men above, whether atheist or not, did not do what they did because of atheism. They did not justify their actions by appealing to atheist teachings. Their religious beliefs can not be directly linked to their unspeakable actions. On the other hand, the abhorrent actions of the religious justified by, or taken "in defense of" their faith can.