"cruel and unusual punishment." is relative. To differnt people it has differnt definitions. In my opinon Torture is not "cruel and unusual punishment" for mass murderers, especially serial killers.
The same thing that putting them in prison achieves. It keeps them out of the general population and harms them. Toture is better at doing those two things. It costs less money and harms them more.
I don't know squat about the US justice system. However, I think I don't need to know a lot to give a legitimate suggestion that we add torture to our forms of punishment for criminals. I wonder if maybe we should torture serial killers or any type of mass murderer or leaders of criminal...