Once again, we have a story about a man who had some sort of psychological issues or mental illness who snapped and ended up killing 5 innocent people plus himself. In this case, the gunman in question seemed on the surface to be quite normal, and no one ever imagined that he'd go out in a blaze of mayhem like this. How can we explain such incidents? Was he evil? Was he sick? Was he demon-possessed?
In biblical times (and in fact in much more recent times and in pretty much every society on earth, regardless of their theology), the explanation for unreasoned evil has been that a force external to the person, a force that is non-material and malignant, known as a demon, somehow took control or gained influence over the person and drove them to commit an act of violence that cannot be explained by the ordinary vices and passions that occur in everyone. We all understand why a person might take a gun and rob a bank, or why she might kill in a jealous rage. But we don't have any explanation for why a person would cut himself, or why he would not only end his own life but plan ahead to take as many strangers with him as possible. We understand selfishness and lack of self-control and greed. But we don't understand pointless evil like this.
So until recently, the best possible explanation has been that there is some spiritual creature, a non-material source of evil, and that this evil power (or even God himself) sends evil spirits to trouble people and to goad them into doing evil, simply for its own sake. People who are possessed by demons might have to be restrained in some manner, and they might be subjected to aversive treatments, or they might be prayed over or exorcised in some way, but their own lack of culpability is relatively clear to most observers.
Demonic influence was blamed not only for insanity and depression in biblical and other societies, but also for epilepsy, deafness, and many other illnesses. Little by little, the biological basis for epilepsy and deafness and leprosy and paralysis and even depression has come to be understood by western science. We realize that people don't fall into seizures because a demon troubles them but because of chaotic electrical activity in their material brain. We know that nerve damage, and not spiritual factors, are responsible for deafness. We know that it is not sin that causes children to be born blind, but various genetic or developmental factors. While we can't prevent all these diseases, we do know where to look for the origins of them, and it's not karma or the parents' sins or anything else, but simply disease processes in the material body.
In the case of depression or psychosis or suicide or other "mental" states, we are a little less clear. We still believe that not only do we have a body, which includes a brain, but we also have a disembodied "mind" or "soul" or "spirit" of some kind, and if it behaves in destructive ways, maybe it is the fault of the person himself, rather than a biological disease. Or maybe, if we are a fundamentalist, we think there may be an evil spirit involved somehow.
One thing that is interesting to me is that "demon" as an explanation works just as well as "virus" or "chemical imbalance." Until recently, no one anywhere on earth had any ability to see a virus or a brain chemical, and so it made perfect sense to think of these as "spirits." They are invisible, they act in arbitrary and unintelligible ways, and once in a while the placebo effect is strong enough to over-rule them, but most of the time, God's will is done and we suffer the ravages of whatever malign forces run amok in the world we inhabit.
If the Bible were actually God revealing otherwise unknowable facts to mankind, I would expect God to have explained these things. On the other hand, if the Bible is a record of some people doing the best they could to grope towards God and towards justice and righteousness and fairness and an understanding of life, then we could expect just what we have--an intuitive guess that some unknown evil power is at work in some people, to be known as demons. There is nothing in the biblical revelation that goes beyond what all people figured out for themselves about the cause of mental and physical illnesses.
So call it a demon or call it an imbalance in dopamine and seratonin, or whatever the cause of such breakdowns turns out to be. Already we have some drugs that can work on these chemicals in a crude way. Some day in the not too distant future, the diagnosis and treatment of brain states that cause destructive outbursts will improve, maybe to the place that the treatment of leprosy and deafness is at today. Another demon will have been driven out.
But it will be human effort, not an act of God, that will have done it. And both the cause and the cure will be biological, material, not spiritual. Even the placebo effect, that seemingly magical ability of the body to respond to worthless cures, is a biological fact about human brains--that the brain's influence on the body is stronger than we thought, that the brain, being part of the body, effects the rest of the body. It is not an ethereal thing called "mind" somehow magically affecting matter. Mind is what brains do, just as heartbeats are what hearts do.