Are you fucking serious? Lethal injection is your idea of cruel and unusual? How about drawing and quartering?
OK look, there is NOTHING in the constitution that says that punishments must be completely free of pain or discomfort. Nothing. Cruel and unusual would mean that the government is going out of its way to punish someone in an especially barbaric method, which in no way shape or form happened with this execution. If the inmate happened to suffer a bit, so what? The state was attempting to carry out the punishment in the most humane and effective way available to them at the time. They never did anything for the sole purpose of causing additional suffering.
If you're going to say that the constitution requires all punishment to be completely painless and free of unplesantries, that would mean eliminating the penal system altogether. Cruel and unusual != any amount of suffering.