Are the mental illnesses preexisiting? or develops within or after leaving the organization ?
I think a little of both. I think the religion attracts people that are struggling, but I also think their teachings cause some people to suffer to a certain extent. I think if you are talking about more serious issues, like schizophrenia or bipolar, then there has to be some kind of predisposition, it's unlikely that the religion could cause it, although it might make it worse. But if you are talking depression, then it's possible they could cause it, it's a religion of guilt. I think in most cases it's a combination of both, nature and nurture.
With my daughter I have to say I can't blame her issues entirely on the religion, she was always going to be different. But when you have a child who worries about everything, having her exposed to the concept of Armageddon on a regular basis was probably not a healthy thing. She was about seven or eight when the Yearbook about the JWs in the concentration camp came out. She obsessed about being put in a concentration camp for a year. But did that cause her issues? Who knows? Most children were obviously not affected in that way. Maybe she was the canary in the coal mine, maybe she would have turned out the same anyway. She also obsessed about growing up and having to pay taxes, she actually cried on her birthday because she had to get older and pay taxes. She's fine now btw.