Capital Punishment - An Alternative for Missouri?

Several inmates sit in Missouri prisons, awaiting a court ruling on if the new lethal injection drug is legal or not. According to this STLtoday article "Propofol has never previously been used as an execution drug, and a lawsuit filed by a death row inmate claims its use could violate the constitutional guarantee against cruel and unusual punishment... Missouri has executed just two men since 2005 as courts have weighed constitutional challenges to the death penalty. Forty-six men are on death row."

While there are continuous debates and lawsuits over whether or not the death penalty is Constitutional, it seems a bit ridiculous that we would worry about if the method being used to deliver that punishment is cruel and unusual. The people facing this dilemma are people that have been convicted of such crimes that legally warrant such a harsh punishment.

So why all the debate over which drug might cause the least amount of pain to the convicted prisoner? While I am not for dragging out the punishment in any way, I do not believe that it is a requirement for said punishment to be painless. With that in mind, I submit another method that is, in most cases incredibly painless, efficient, quick and is ridiculously inexpensive to perform: hanging.

According to Wikipedia, in New Hampshire, correction officials can decide to hang a prisoner as the method of capital punishment and in Washington state, the prisoner can still elect to be hung as the method of choice. Is there any legal reason why this method could not be utilized by the state of Missouri to deliver the desired outcome of such rulings? The immediate obvious answer is "that's barbaric and we are more civilized than that". But is that true?

What we are talking about here is the predetermined killing of a person who, in most cases, was convicted of a crime by a jury of citizens that decided their punishment should be death. So does it matter if that person falls asleep after a few minutes of drug injections or a few seconds at the end of a rope?

Wouldn't the more human method be the quicker method? After all, who can say what thoughts or dreams may be caused by the drugs in the moments prior to them actually killing the prisoner? After all, it is not beyond the realm of possibility that in those final moments we could be subjecting the person to and incredibly terrifying and painful dream.

But what about the possibility of  a botched hanging attempt?
It appears that the variable drop method of hanging, where the prisoner's height and weight are used to calculate the drop distance needed to deliver enough force to snap the spine of the prisoner, has been used successfully for many years, many times, with few instances of a botched attempt. In most cases the prisoner is immediately either killed or knocked unconscious until the deprivation of blood to their brain kills them a few minutes later.

I am far from an expert in this area and am not outright endorsing such a maneuver, but wouldn't it be to every state's benefit to seek out all alternatives and offer cheaper ones to prisoners as a choice (as Washington currently does)?

It's puzzling to hear such rigorous debate on what constitutes a human killing, when the person being handed the judgement did something that was very much far from humane.