Eighth Amendment protects us from a cruel and unusual punishment. Once we start saying "fuck that let's kill them all with a hammer" where will this end?
Please note, the argument is not about death penalty -- it is about lethal injection. Such arguments have been brought up before the Supreme Court before and I do not see what is wrong with it. As societies and people evolve, our norms and laws have to evolve too. Questioning and changing our laws is not the end of the world.

Death penalty itself was a subject to questioning because the wording in the Constitution is loose. I highly doubt that we can have a decate w/o case where somebody challenges it. What is cruel? What is unusual? Does capital punishment fall into that category? If we do something more often will this become usual and not cruel? I am sure people will figure things out.