Good and Evil and One God.

beliefnet

If they are athiest murders they are clearly INSANE.

If they are christian murders its the religion's fault, - RCCU

With or without religion, good people can behave well and bad people can do evil; but for good people to do evil — that takes religion. - Steven Weinberg

  Good people which are the overwhelming majority of humans will behave well that is they will conform to the prevailing ethos of their community.  There will be a few sociopaths sane or insane that will violate the norms of the community, but normally they will be removed quickly by the good people in the community. 

It takes a strong belief in the doctrines of a "god" to change the prevailing ethos of the community to promote or tolerate evil.  The religion may be secular in that the "god" is human but it takes a well buttressed belief system in something or someone promoting that something to allow good people to partition off the evil in their minds.  

Miniver Cheevy

A group of people no matter how committed can never accomplish anything.  History shows that on any scale, large or small it is always a single committed individual that for good or usually for not so good can feel the prevailing needs of a large group of followers and provide a focus for their beliefs. Whether it is a genetic imperative as some argue, or a conditioned belief like the major religions, without a committed leader nothing happens.  The leader may be supernatural, which is the power of religions, or human, but H. Sap. need to believe in something or someone to do anything.