The bullshit way to think about demon possession vs. mental illness goes something like
"The ignorant medievals/savages/evangelicals think that they have demons, but what they REALLY have is schizophrenia! We've solved this problem with cold secular Western empiricism! Submit to the DSM and be cured!"
Like, if someone comes to a healthcare practitioner and says they have demons, the response should be more like "what problems are the demons causing for you?"