Two conclusions are inescapable....
Have you conveniently forgotten that atheists killed more people than religionists by a substantial multiple, last century? Stalin and Mao alone evened the score for a large portion of human history. So much for "reason".
1: Many people have killed many other people.
2: Many people have religious beliefs.
Any conclusions beyond these two are unsupported BS.
For the record, what all the mass murderers have in common is a willingness to classify people into arbitrary but reasonably easily delineated groups and declare members of one of the groups to be "unclean". The stated goal is inevitably the betterment of "mankind" ignoring the fact that mankind is made up of individuals each "created equal" with an equally sacred stake in living, loving and raising families who share their goals and values.
If you are among those, willing to condemn such a group to the point of wishing death upon their members, then you share in the evil that the mass-murderers represent.