Unfortunately the death penalty is currently not about killing the criminal, but a good-looking death. Basically it does not matter if the person suffers for an hour, as long as it is not visible.
From hanging, electrocution, to gassing (yes, US did that at one point) and lethal injections, the method was always slow and painful.
With the lethal injections one, most chemicals were banned for it, so they are poorly researched by laymans, not as functional and literally made so the person is paralyzed, while they still can feel everything and can painfully be dying for half an hour.