There is no doubt that solitary is cruel and can have devastating effects on those who suffer it -- particularly the mentally ill.

But without a substantial investment in mental health services, as well as a viable alternative to protect both corrections staff and other prisoners, it may be a necessary evil for the time being.

The mental health system has failed the populace at large, not just inmates.  There are neither enough beds nor enough effective programs.  And should the rights of a violent inmate trump those of the ones s/he threatens?

A better alternative may be to increase the level of human contact without allowing direct physical contact -- i.e., cells with bars rather than walls for a good part of the day, and more opportunities for exercise and fresh air.