I'm not being clear, sorry. Using Occum's razor, how did you decide this was the result of someone being mentally ill (when the bible's version of god routinely has humans do his killing for him, so there is precedent) and not divine command?
Applying Occum's razor to this case for me would look something like:
"The perpetrator claims to have received a command from god, much like the one the bible says he issued to Abraham about Isaac, or any of the other killings god commanded humans to perform).
THeory #1: creator of the unvierse commanded this guy to murder people. Okay, so first thing is can I show that there is a creator of the universe? Well, not really, so I would have to assume there was this entitity, and THEN would have to demonstrate that said entity, again undemonstrated, is actually this specific book character. And THEN I'd have to find a way to confirm (a) this universe creating entity requires humans to do its dirty work like murders even though it'd be powerful enough to create a universe and (b) confirm its communication of these requests. Because the believers in this particular entity seem, in spite of the character's behavior in the book, completely convinced that this god would never ever command someone to go murder a baby, then, we'd have to figure out why he got it wrong, or if he was mistaken. If he's MISTAKEN about the voice, what's that tell us?
THeory #2: he has a mental imbalance. We know there are people with chemical imbalances in their brains or traumatic brain injuries, because we see them every day, they're studied and documented and often treated. We know some of those imbalances, left untreated, can lead to delusions like hearing voices that aren't there or seeing images that aren't there, and we can tell that because only the afflicted report these beliefs. Oftentimes these imbalances lead to violent behavior.
Does theory number 2 seem applicable even if theory #1 is completely deleted from this post? That's Occum's razor: the theory that adds the least complexity is often the correct answer. But for a person who believes in god as you do, you cannot delete theory #1, so you're kind of stuck working with that theory until it's proven false, right?