If that were only about LWOP-able offenses, that would make sense, but trying to deal with a serious mental/nervous [take your pick] illness in a jail or prison rather than a hospital, in the case of people who will get back out sooner or later, is incurring needless risk of trouble for society.
For the current state of incarceration, I'd probably agree with you. But if we're assumming that no cure is available for the heinous offender, then wouldn't the right of citizens to be free of criminals trump the welfare of the offender?