I think the stigma against mental illness exists because mental illnesses tend to be more challenging to interact with, on a personal level, than for example, diabetes or hypertension. It's just not the same, if we are honest about it. It's more challenging to have a conversation with your average psych patient than your average diabetic. At least it is for me. It's one thing to go over blood glucose levels with a patient. It's quite another to deal with a patient with major depression who repels others with a blanket of doom. It's one thing to run into your neighbor and hear about her arthritis. It's quite another to run into your neighbor who's manic. Again, this has nothing to do with excusing our responsibilities to do better and meet the needs of those suffering from mental illnesses. However, the analogies to other illnesses like diabetes, etc. just fall flat IMHO.
This isn't an excuse, though. We have to do better. I doubt the stigma in our culture will fade very quickly, though. And to really delve into complexity - what if a mentally ill person is just plain mean, and they were mean and nasty before their mental illness? Or if their mean behavior cannot be separated from their mental illness? These things can get very complex. I don't know the answer regarding the shooter mentioned in the OP. If he was that psychotic, he may not be culpable. If strong psychosis is present, it may render the "downright mean" issue moot.
I dealt with depression years ago. I don't ever want to go into that pit again. Right now I am staying with my mother (and her personality disorder) while I recover from a major surgery. I don't know how much more I can take. If I could physically drive for hours and hours to get to my home in another city, I would. I believe this is why the stigma persists; it is psychologically taxing to deal with those with mental illness. Again, not an excuse for our culture not doing better, though.