Atheism doesn’t lead a person to commit mass murder. Kelley was unstable way long before he found atheism.
You don’t have to believe in God to be a good person. Some people though turn to the dark side.
Why, no one knows.
While atheism doesn’t lead a person to commit mass murder, it certainly removes huge obstacles. There is no doubt in my mind that much of the worsening depravity we see today is a direct result of Judeo-Christian values being removed from the public sphere - not just official government stuff, but EVERYWHERE. I see the attempts to minimize the crimes of certain “preferred victims” (homosexual deviants with sexual contact with minors, Muslims with spousal abuse) as further eroding a common set of values that make a society function.