I could point you towards a lot of info on demonic possession. There are certain traits that differentiate it from simple mental illness.
But why would I bother? I don't think you understand evil very well, perhaps the jump from simple evil to possession is a bridge too far.
ETA: I just find it astonishing that you haven't met anyone who was evil.
Tyrone;
Good morning to you sir, I hope the day in your part of the world has been seasonally appropriate and you're well.
Whenever this topic comes up, I'm reluctant to wade in, because I'm acutely aware that there are personality types who will not believe in certain things until they witness them personally.
Even then, for some, being front and center to events that have no scientific explanation, defy logic and plain don't make sense will not be sufficient for those folks to make that leap of faith.
Emphatically, I get that as I was one until I experienced something that couldn't be explained, then had to come to terms with it.
Regarding demon possession, I've had two different close family members involved in two separate occasions where the individual was either possessed or through unknown means exhibited super human strength, was able to radically change their voice and knew details about other participant's lives that couldn't be explained.
Again, I am not trying to convert anyone and fully grasp that some will not be able to believe or accept what they see and experience.
Many roads to Mecca and all that, plus we're all made a wee bit different aren't we?
All the best to you this last week of the first month.
Dwayne