The death penalty is wrong, being irreversible. However, I'm not going to apply that to the past by saying the inventors of the guillotine or electric chair were wrong. Everyone's at least partially a product of their times. If we hadn't had those methods, would execution now be outlawed in nearly every civilized country? Impossible to say. In any case, even 30 seconds more of consciousness, though gruesome, is less painful than being flayed alive, thrown into boiling oil, drawn and quartered, burned alive, or, if you were very lucky, choked to death or beheaded incompetently with multiple strokes of a sword.
Would as many people have died during the French Revolution without the guillotine? I happen to think so. They probably would have been burned at the stake. The "enemies of the Republic" were so dehumanized by society, anything that was done to them was considered perfectly fine and wonderful entertainment. People were used to seeing horrific executions for traitors anyway. I'm sure they considered themselves civilized for simply beheading people; and, in a way, they were.