so wretchedly bad at it that you feel, if you are capable of such feelings, sympathy for a human being suffering the state of a too-long death--no matter what that human being had done. The demonstration of humanity isn't for the benefit of the murderers--it's for ourselves.
There is a point there--a bloodless death by poison has a lot to do with the conscience of the viewer, that might recoil at the sight of blood or the the blackened face of suffocation. It is possible, nonetheless, that this pang of conscience exists because so many of us have the idea that killing isn't right. Sanitized with a medical display of nice clean chemicals, we can imagine that we are euthanizing a monster so he is no longer a harm to himself or others, not that we are cleansing the world of one terribly inconvenient sinning person. Who is, after all, in custody, and likely will not harm a soul so long as he is.
Or that's how I see it. When an execution goes badly, it reminds us of what it is--the planned killing of another person. A crime, if not done by the state, that many states punish by death.