This sounds like a joke but this is the explicit problem: doctors won't be the ones to do it.
You guys all knew that right? Doctors don't administer those chemicals for lethal injection. And they won't be administering gas either. Some po'dunk cop will.
Because doctors take an oath that begins "first, do no harm". This has forever been the problem of the very notion of "humane execution", there are no physicians involved. None. At any step.
Know what's just as effective? Bullets. But we can't call a firing squad humane with a straight face, and the witnesses remaining are traumatized, including the shooters. That truth exposes the truth of the death penalty. It's not about justice, but retribution - for the living. They're lynchings. Violent theatrics. That's the point.
They shouldn't be legal, it's barbaric. But you already said you weren't for them, so I'm just preaching to the ether.