Everyone knows there is no monster under the bed. Come now, you are sensible: you know that monsters are only a harmless metaphor. Same with demons. People aren’t possessed by demons; we all know that. Our ancient forebears did not know about mental disorders caused by chemical processes in the brain. They didn’t know that everything can be explained, in due course, by sound research and careful study.
The medieval Church, led by shrewd theologians like Thomas Aquinas (who read his Aristotle with understanding), slowly created what we know as the university: a center of study and inquiry that teaches us to trust the ready evidence of our senses. When I worked as a therapist I often reminded myself that all behavior — no matter how awful, strange or exasperating — all behavior makes simple sense. If a couple’s marriage is collapsing around them, their dilemma was caused by ordinary circumstances and events, not mysterious or monstrous forces. If a person seeking therapy is depressed, and another one is anxious, and yet another has anger problems, all three have readily explainable challenges, not demons. They’ll benefit from realistic and skillfully designed therapies.
In short, no, there is no monster under the bed.