i dont see what needs to be explained. religion has killed more people in the course of human civilization than atheism.
religion has been used to dehumanize opponents. so has atheism.
religion has been used to justify horrible crimes. so has atheism.
two differences.
religion has been at it for a lot longer.
and those crimes commited by atheists were not done in the name of atheism, more like a political expediency.
i guess any philosophy can be turned to evil ends, its just that religion has the distinction of having that done to it longer and with greater severity than atheism.