I’m probably going to take some heat for this - but I am compelled to tell the truth...
There is no mental disorder that CAUSES a human being to take the life of another. On the other hand, one thing can (and I will admit it is possible that mental disorders can leave a person more susceptible to it) - and that is evil. Evil in the form of demonic influence.
The governor of Connecticut said it in a somewhat veiled way when he said evil visited this community today.”
Yes, there are diagnosable, and measurable chemical and physiological issues that cause a myriad of irrational thinking, learning and behavioral complications, and even intellectual deficiencies. But those are a distinct issue in contrast to the evil that causes one human to slaughter another.
I agree with you.
A paranoid schizophrenic that murders someone because voices in their head told them to, or because the see the person they murdered threatening them with an non-existent weapon, is hearing and seeing those things because of a measurable chemical or structural disruption in the brain, most likely caused due to an inherited gene.
There's no reason to appeal to a supernatural explanation for something when there's a clear science-based explanation for something.
We don't need to come up with theories about demonic possession to explain mental illness or "evil" actions resulting from mental illness any more than we need theories about an evil dragon eating the sun to explain eclipses.