I don't think lethal injection is the soft kneed method of killing people. Capital punishment is about issuing a sentence to a crime. The quickest most humane way is the likely the best way, as it causes less controversy. Look south of the border at when the debate arises about capital punishment. It is almost always when something goes wrong and Joe Public is exposed to some horrific scene of someone dying a horrid painful death (even if they do deserve it for the crimes they committed).
If we really wanted retribution, we would allow what we think of the most heinous of criminals (child molestors, psychopaths, sadists, etc.) to live among the general population for as long as they could survive.