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.