I would suggest there is a spectrum between a frame of mind (mild affliction) at one end and a nasty, dangerous separate personality
huh? I don't understand how there can be a spectrum, a sliding scale between being upset by past events and being haunted by a supernatural personality. Please explain that further.
Have you ever had an argument with someone (quite often family) where the two of you went off the deep end ? and afterwards (perhaps the next day) both of you possibly thought that was a bit nuts ? Or perhaps you've witnessed it with two people you know well ?
Where the argument, fight got way out of proportion for what seemed to have been just a minor trigger ? Where the 'combatants' start bringing up every past negative event they can remember to 'throw' at the other person ? Sometimes these are mostly one sided fights. But often one party going off will set off the other one.
Individuals afterwards when things have calmed down, can often be sincerely shocked at things that he/she said or did. 'I was not myself', 'I don't know what came over me', 'I'm not sure what possessed me to have....' are some of the phrases that you might hear.
For anyone who knows a bit about Eckhart Tolle, his name for this phenomenon is the 'pain body' which we each have inside us. A 'garbage bag' full of hurts and resentments which lives on inside us, often dormant and quiet, but can rise up on certain occasions, if certain triggers arise, if certain buttons are pushed. Depending on the individual, the 'pain body' may be relatively harmless or may be a dangerous beast, in which case when it awakens, physical violence and harm may result.
In extreme cases, people have to be locked away, either because they have injured people or because they regularly lose grip on reality in dramatic ways.
The word 'supernatural' does not need to be used. However if you witness an extreme case you may see actions and appearances which stretch the boundaries of ordinary explanations. I don't feel a need to call it supernatural. But, I would say that there is a whole lot more to 'natural' than many of us think.