This would all make sense if there were any evidence that the use of solitary confinement actually reduces prison violence. But it doesn’t. It doesn’t deter misbehavior or violence, it actually increases recidivism, and it is not used only against the “worst of the worst.” Where solitary confinement has been reduced or eliminated, there have not been corresponding increases in violence.
Given the complete lack of evidence that solitary confinement serves any legitimate correctional function, it is nothing more than meaningless torture. Obviously, the Eighth Amendment prohibits torturing prisoners in all circumstances, but the prohibition is particularly stark where there is actually no benefit to the practice from a security or recidivism perspective.