How do Atheists and Theists respond to these atrocities?

Atheists insist that the world would be a better place without religion. Theists insist the opposite. Atheists cite the horrors of the crusades, and the Islam extremist groups as the violence that religion causes. Theists can cite the campaigns of Adolf Hitler and Josef Stalin as what happens when an Atheist takes control. Which side is right? Or does religion/lack of religion have nothing to do with violence?