When it comes to understanding strange behaviour there is plenty of controversial speculation about how the Devil is involved.
There may be two extremes in this regard—one that does not accept the reality of evil and the other that ascribes too much blame to dark forces.
I remember a man who worked with people with exceptional needs telling me that he thought a particular individual must be possessed to make such strange gestures. Out of his need to explain the unexplainable, he gave the Devil credit.
We see the word lunatic used in the King James Version of the bible to describe some people that Jesus encountered.
If we accept the reality of evil forces at work in the universe, we must ask how they affect humanity. The Scriptures do tell us about the Devil’s strategies. Jesus described himself as a shepherd who stayed at the gate of fold to protect his people from the Enemy.
Peter, who was once warned by Jesus that Satan wanted to sift him like wheat, later was known to say,
Peter knew first hand how the enemy could affect the mind with crippling fear. Peter knew how the mind could play tricks on you and how the devil could convince you of great falsehood.
It is not fair to say that a mentally ill person is possessed by the Devil. But, it is very possible that in the vulnerability of their condition, the Soul Enemy goes to work to bring a crushing load of accusation, shame and destruction. The debilitated mind is not in a state of sobriety where it can process thoughts in a healthy manner.