At the time the Bill of Rights was written, the authors specifically sought to ban such execution methods as burning at the stake, crucifixion and breaking on the wheel. In modern times, the Supreme Court has decided cases that redefine what the Founders meant. In Hudson v. McMillan (1992), the court ruled that the use of excessive physical force against a prisoner might constitute cruel and unusual punishment, even if a prisoner does not suffer serious pain. But the actual infliction of physical pain or hardship is not necessary for such a finding. As far back as 1958, the Supreme Court ruled in Trop v. Dulles that the use of denationalization (the deprivation of citizenship) is a punishment barred by the Eighth Amendment.
Aside from the period between 1967 and 1976, when capital punishment was effectively suspended, the Supreme Court has consistently ruled that the death penalty does not violate the Eighth Amendment, but that some applications of it might. The Court declared the execution of the mentally retarded to be cruel and unusual punishment and, thus, barred by the Eighth Amendment (Atkins v. Virginia, 2002). In Roper v. Simmons (2005), the court ruled it was cruel and unusual punishment to put to death anyone who was under the age of 18 at the time they committed their crime.
I don't know how you define cruel and unusual in a lethal injection case. Angel Nieves Diaz was said to have a physical condition that required more drugs to kill him than if he had not had the condition. If those administering the drugs had known about it and given him a double dose so he might die within the "norms" of such executions, would that have been constitutionally acceptable? Does this not get us into the same arbitrary standards that are applied to the unborn? At first, the Supreme Court imposed an arbitrary trimester standard, forbidding the state from restricting a woman's decision in the first three months. But subsequent rulings have resulted in abortion on demand, for any or no reason and at any time.
Will the same erosion of justice against convicted killers mimic the erosion of rights for the unborn innocent? The arbitrary way in which we approach anything of importance today would suggest it might.
To avoid this legal hair-splitting, why not return to an earlier and acceptable method of execution that ensures justice is done and inflicts minimal pain on the guilty: the firing squad.