I believe that people, when in a weakened state, can be used by evil to do horrible things, yes. Those who let evil in may not find it easy to kick the evil back out.

But that isn't to say that I think people with mental illness are necessarily possessed, even if they think they cannot control their actions. I think there is a big difference between evil and illness.

The people I have known who did really bad things were all conscious they were doing bad things and did those things deliberately. Every single one of them justified what they did, mostly through blaming whoever they victimized or minimizing what they were doing or had done. One of the biggest bullies I know justifies what she does by denying that (read any list of bullying tactics) is bullying. In my opinion, she's given herself over to evil and I doubt she could shake it even if she tried.

I think there are reasons why we should try our best to keep ourselves mentally and physically healthy; reasons beyond what we even understand. When I meet people I think would qualify for what could be called possession, I stay very far away from them, psychic-ly speaking.