I think far too often, in our sophisticated society, we overlook this distinct possibility in cases of horrendous crimes such as this.
No one can say "the devil made me do it" as possession by demonic forces can only occur, if a person gives permission...for those things to take control. None-the-less spirits of evil simply want to destroy people, and will push willing-victims into mental illness, and will use such victims to harm and even murder others.
Here's a good summary of what is translated "demon possession" in the bible. I know nothing about this guy's other teachings, but this post is sound.
Thanks