I believe it is just as immoral to use mental illness in a derogatory sense, just as is it to exploit your own mental illness for personal gain. However, to keep dangerous people off the street is not a "crossed the line" moment. As someone who speaks from experience, there are moments where one needs help and requires special care in order to get better (and off the streets) and then there are moments where you are well and your disorder is not creating a hinder on other people. There is also a fine line between those who are mentally ill and those who are indeed "nutcases" that are high risk individuals, or their own beliefs encourage such bad practice in others where they don't have control over their own illness. A "nutcase" who will be renting out a van to run over innocent people on the streets is a perfectly valid person to keep off the streets. To then exploit your own mental illness to digress from this fact is just as sickening (if not even more harmful) than someone making a derogatory statement.