No! Pinning it on "theism" at this point is not appropriate. We don't have enough information. You very rightly express care and concern for those who lost loved one, and the parents of this killer live their own private hell that can never be erased. Maybe they had a hand in making this young man do insane things, but the reality is, his body may be overloaded with testosterone, he may have had some shock to his cognitive system, he may have some abnormality in his brain ... we just don't know.
Part of my training was watching interviews of serial killers (Jeffrey Dahmer, Ted Bundy, etc.,) as well as interviews with their parents, teachers, physicians, and there is not a single recipe for making an insane person.
To Loren: Hear, hear! The rational mind wants to find, needs to find the triggering event that bifurcated the killer's brain between outward normal behavior and inner madness. If, while blasting away, he'd screamed, "Die and rot in hell, all you unbelievers!"...we might have a clue. For all I know, his PhD hassles drove him over the edge (PhD programs have destroyed more than one life).