Smith clearly died in agony from this method. Lethal injection was also promoted as painless, which it would be in theory and was not in practice for the same fundamental issue that the whole death penalty process involves incompetent people fucking up because competent people won't take part in it.
I can't pick a method when it is guaranteed to be horrible because of the context of the death penalty.
I will gladly pick a method or two for people who want to be euthanized and participate willingly. Those same methods will always be torture to someone who does not want to die as long as incompetent people are running the show.