Until Death Do Us Part

"But until [today], the justices had never agreed to consider the fundamental question of whether the mix of drugs used in Kentucky and elsewhere violates the Eighth Amendment's ban on cruel and unusual punishment."

My legal brain is confused on this one. If lethal injection is cruel and unusual, then what type of government imposed death is not? Firing squad? Hanging? The electric chair?

If I had my choice, I'm picking the needle.