Abuse of religion is not excused by citing people who don't adhere to one.
Using religion to justify violence against others is an abomination.
It undermines the most important function of religion for the majority of believers, which is to provide a context for human relationship to the transcendent, by turning religion into a kind of us-vs-them team blood sport.
Famous atheist (or, in Hitler's case, pagan) brutes also earn reproach, but are beside the point in a discussion of religious violence committed by religious people for so-called religious reasons.