"a semi-autonomous personality complex split off from the ego as a consequence of trauma"

Yes, that's another possibility. Or...it's another way of verbally describing the situation.

The point is, the person is mentally disturbed, correct? Now, every society (and every person) will find different ways of responding to that. Some of those ways may be helpful, some may be deeply harmful.

Possible ways of responding to a mentally ill or disturbed person:

1. Talk to the person about it. Counsel them. (probably harmless in most cases)
2. Drug them. (a very popular method in our society...often very harmful)
3. Throw them in an asylum and let them rot. (used to be popular not long ago...very harmful)
4. Beat it out of them. (definitely harmful)
5. Pray for them (Almost certainly harmless. Possibly helpful? Not for me to say, is it? Not for you either.)
6. Kill them. (a quick solution in many past societies)
7. Make up your own method if you can come up with one...

My point is, the mentally ill have often been mistreated in most societies, including our own present one. It is very common for horrors to be perpetrated on mentally ill people whether or not they are said to be "possessed by demons". You can look up an example of horror in the one case, I can look up an example of horror in the other case. So what? It proves nothing final about the matter one way or another...it's just one more example of horror in a world that has always seen a great many horrors.