This is a sticky wicket, pf. I can't remember the statistics, but a study I read said that most people in jail qualify as having a mental illness, but the term is often used for convenience, as generally people, good or bad, are often only considered mentally ill when it's an inconvenience.
I do believe that rehabilitation and therapy are the most productive ways to help criminals, especially first-timers, rather than just putting them in the Bedlam of jail to become worse, only to release them as hardened criminals, but I'm also very aware that where you get to know the person, as with programmes like this (I recently saw a programme about young offenders), can induce a sympathy at the expense of the people they may have damaged or destroyed. Some people react to the things that caused their issues with aggression. Others become insular. The aggressors can cause enormous damage, especially to the insular, who often become their targets. When I watch these programmes I think of the quiet people, the ones still suffering in silence, they have harmed in their wake. Then their on screen charm doesn't seem so pretty. Some people are very ill to the point of not having awareness of what they are doing, and I appreciate that, but whether they can be rehabilitated I guess depends on the strength of the crime, and whether there's any coming back from that. Being sick in the first place is not that person's fault, but some people go on to do terrible things, and some go on to do good things. Perhaps we can only do our best to help those that can be helped, while protecting society if they are a risk.