If you know anything at all about the nature of spirituality you are aware that it is fundamental to the nature of reality. Mental illness as well as mental health are functions of spirituality. The simplest way to understand this is to recognize that love, hate, greed, generosity and countless conditions are of a spiritual and not a physical nature.
Demonic possession is domination by destructive forces. Salvation is domination by Godly forces. They are not equal but they are unique from each other. These are the principalities spoken of in the Bible. Acknowledging these forces is central to overcoming demonics.
The killer in Texas was an avowed atheist. He did not acknowledge the reality of God consciousness but he did open himself to destructive, evil forces. That is, while he rejected God he clearly did not reject Satan’s influence.
There, fixed it fer yuh!
Seriously: Your comments are thoughtful and insightful. If there is a spiritual component / factor, it is more subtle than the psychological stratum, and might underlie it. The psychological angle, however, should be considered first.
Regards,