Not many get the comfort of dying in their sleep, murders should be executed in the same manner they murdered the victim/s.
Of course that would be cruel and unusual, as if the victims were all murdered comfortably in their sleep.
Applying the rules of statutory construction, “cruel and unusual” means a prohibition to a punishment that meets both definitions. Otherwise they would have written it as “cruel OR unusual.”
So, putting the condemned to death in the manner in which they killed their victims may be cruel, but it’s not unusual if you do it to all of them.
I don’t see the constitutional problem here.