In ancient days, they burned "witches" at the stake. Later, criminals met death at the guillotine. But it was all so unsightly and disturbing for the witnesses.
Then there was hanging. If done right, the neck would break instantly and unconsciousness would occur. But it was still so unsightly and disturbing for the witnesses.
Firing squad? Messy. And still rather unsightly and disturbing for the witnesses (although it was used as recently as June 18, 2010, in Utah).
Then someone invented the electric chair. If done right, the body might not catch on fire and you might not smell the burning flesh. But it was still so unsightly and disturbing for the witnesses.
So they invented the lethal injection. An IV would be inserted in a medical-looking setting and the condemned would appear to go to sleep. Very peaceful-looking. And much less unsightly or disturbing for the witnesses.
But not necessarily so for the condemned.
The most common lethal injection formula used in the U.S. consists of a three-drug protocol:
1. First, sodium thiopental renders the prisoner unconscious.
2. Next, pancuronium bromide, a muscle relaxant, is introduced, causing complete paralysis.
3. Finally, potassium chloride stops the heart.
Unfortunately, there is reason to believe that the sodium thiopental doesn't always work as intended, meaning that the condemned may actually be conscious as the potassium chloride burns its way through his bloodstream to the heart. But the prisoner cannot express the excruciating agony he feels because that second drug, the pancuronium bromide, has him paralyzed.
And all that, of course, depends further on the execution staff being able to insert the IV. Many prisoners have damaged veins as a result of past intravenous drug use, making it difficult to find a usable vein. This prolongs the pre-execution process and creates more pain and suffering for the condemned. Of course, the witnesses don't get to see this, because the curtain on the death chamber window does not open until the IV is in place and the drugs are about to flow. This way, it's very peaceful-looking. And much less unsightly or disturbing for the witnesses.
The benefit of lethal injection, therefore, at least to the extent that it is shown to the public, serves only the witnesses' eyes and stomachs.
If it looks peaceful and painless, it's much easier to watch, even if it's only an illusion.
And, if it looks peaceful and painless, it's much easier to justify, even if it's only an illusion.
Take away the illusions and you have the very real possibility of cruel and unusual punishment, which is supposedly forbidden per the Eighth Amendment.