Stigma exists largely because people don't understand mental illness, and they fear it. Also, for example, one's osteoarthritis does not result in a person glaring at you or acting "bizarre" like mental illness can, forcing uncomfortable interaction.
Just because someone has mental illness does not mean they aren't responsible for controlling their reaction to it as much as they are capable. Shades of gray is what I am talking about. Our culture isn't so good with complexity.
I have seen some with mental illness reducing their responsibility to taking a pill and disregarding their responsibility for managing their reaction to their illness (behavior) within their capacity. And I've seen others nearly destroyed by their illness despite trying very hard.
Hopefully we can increase respect and maintain complexity of thought regarding those who suffer, which is most of us at one point in our lives or another.