To be honest, I’m not against solitary confinement, for the many reasons expressed above by those who are for it. If you did something criminal/illegal, then punishment is the answer. Punishment is not meant to rehabilitate nor is supposed to be fun. If you are a danger to other prisoners now that you are locked up, you not only deserve your original punishment of being locked away, but it but it is necessary that you be confined alone.
As for those who turn “insane”, I think that if you are going to murder little children or do some other sort of terrible crime, you aren’t all there in the first place, so your sanity is already diminished. And who is to say that God does not test these people, who rightly deserve to be locked away, have failed their tests at becoming less violent? He gives everyone free will and theirs accordingly was enough to reasonably be put into solitary confinement.