On the one hand, the largest murderers in human history, the communists, were not doing so for religion (though they treated the state and "dear leader" as a godlike figure). On the other hand, if you go farther back in human history, the percentage of people that died to human violence was FAR higher than it was even in nazi germany or the soviet union. Far higher.
Studies of primitive tribal peoples show that more than 25% die to homicide. Those people ARE religious (and one religion is just as likely as the next in terms of veracity—if you disagree, I'll happily use whatever algorithm you use to throw out tribal religion on yours
). The decrease in human violence has nothing to do with improvements in religious invention, either. New Testament era people I'm sure died at human hands at a higher rate than the 4:100,000 we might see today.