Bro if my life ends by being decapitated by a machine perfectly engineered for decapitating, that's a plus! Not everyone gets to go out on such a high note. I'd be so down with that shit so fast. Fucking decapitate me! Not with some drunk ass brownoid swinging a sword, but with a perfect machine.
Real talk... I think this is a big reason why people get so weird about it. They don't want the bad guy getting his swan song moment, stared at by the world as it stops, even if only slightly, just to pay attention to him. They might not know that's what they want, but this sounds very right based on all the criticism of the penalty I've heard that wasn't about the procedure to get there.
Guillotine? It's innately flashy. Big blade, falls down with a noise, some dude needs to sit there and watch you to make sure you don't do something and yank your head out, and your blood's gonna spurt out of your neck hole and leave behind a nice trophy.
Hanging? Same deal. You need a whole ass platform, a showy drop with sudden stop, and if the guy fucks it up you might actually still live! If he doesn't, the crowd gets to hear your neck break!
Electric Chair? Nothing sparks like electricity baby! Your body is probably gonna spazz out as the machine makes a big noise! Maybe you get the movie thing where the lights flicker a bit too!
Lethal Injection? They gotta hook you up to a machine and take precautions so you die properly! They're definitely giving you attention!
No one's bothered to frame the death penalty for what it is, the liquidation of unsalvageable and unredeemable human body parts, and sell that to the public with a disposal method to match.
I think the people want something with no pomp. No circumstance. Just carted off to a closet for whatever is meant to happen to happen, blindfolded so you don't see it coming and can stew for a bit, and then life goes on after the janitor cleans up whatever mess is left. Something degrading in that special way to signify how no one actually cares, not that the state or whoever cares so much they're killing you.