The difference between a mental illness sufferer and a jackass

When I get into online conversations, something happens about once a month. Either in an attempt to be compassionate or to condemn, somebody will declare that this corrupt politician, that creep, this annoying person or that wrong-headed one, is probably suffering from a mental illness. Inevitably the person making the comparison doesn't actually have much experience of mental illness, and consequently knows as much about it as anyone who hasn't studied it as a professional or encountered it ...