It's not possible to leave your soul behind anywhere. For any reason. I believe such a statement is not meant to be literal, but rather a way to express the severity of what they were exposed to.
As far as any demonic influence, it doesn't take PTSD to be influenced by demons. Everyone already is.
The way trauma works is it gets stuck in a part of the brain and the brain cannot process it. When you sleep your mind processes your recent memories. This allows them to be understood as a past event.
With trauma that memory stays in place, rather than being processed. This prevents the brain from cataloging it and instead when it comes forward, rather than being a memory it places them back in the place where the trauma is. They are not just remembering it, but reliving it as if they were there again.
Imagine taking the biggest extremes of fear fear and violence and having to relive those moments again and again. Being randomly triggered without warning. Often disrupting sleep, leaving one tired often.
Now imagine the physical toll that takes on someone. Put all that together and imagine the mental and emotional toll that takes on someone.
Their souls aren't left behind. Their experiences are just that devastating. And few ever get treatment or help so it continues for years. That drains a person. Changes them.
