Religion is mental illness. Being something like a Jehovah's Witness forces you to turn off and rewire natural empathy. People who commit such heinous crimes as this also lack empathy. He may have no longer been a witness, but with filling a child's head with stories of the flood, the hebrew genocide of canaan, losing family and friends at Armageddon, can disrupt "mental health". With this person we may never know if the witness environment had anything to do with exacerbating his (to quote simon) asshole nature.
Any time you can override or ignore your empathy for another innocent human to kill them in this psychotic premeditate way, I think its safe to say you have a mental problem, from within, or from an institution like a religious or militant cult.