It just occurred to me recently that religion is probably the biggest player in stigmatizing mentally ill people. To me, a naturalist, mental illness is just like any other illness - why would anyone think badly of a sick person? Seriously, people don't stigmatize people who suffer from cancer or kidney disease, do they? Why think badly of people for having another aspect of their body go haywire?
The only possible reason I can think of that people would be judgmental of mentally ill people is that those judging believe a person's "self" is something separate from his body. I'm surprised I hadn't thought of this before.
When going through rehab after a serious head injury I heard many conversations about other people worse off than I was. One conversation that particularly stuck with me was about a man who been in a car accident that damaged the frontal lobes of his brain. The conversation I overhead was actually frightening to me. Two of his family members were talking about him. One said to the other that Bob (I don't remember his name so he's Bob) got angry a lot and tended to use a lot of profanity and at other times acted sullen and childish. His other family member responded that Bob needed to get right with god - as if poor Bob's problem was a crisis of spirit rather than a result of the damage under the plate in his skull. It also made me very sad. That wasn't a very loving attitude to have towards Bob.
I encountered this attitude a lot, as if the "soul" is real and responsible for the mind and that because of this one could simply decide not to have mental problems. It's a real bitch when you are faced with brain damage and idiots who think you can just will yourself better. I think this idea of a soul separate from the body is responsible both for people's weird idea that once your hair grows back after brain surgery you're all better and the idea that mentally ill people are bad and choose to be sick.