But any feasible execution method will be imperfect, subject to equipment malfunction and human error, and arguably inferior to available alternatives. The Supreme Court has not ruled on the constitutionality of an execution method since it upheld the use of firing squads in 1878. If it endorses the Eighth Amendment standard urged by Baze and Bowling, it will invite endless litigation over the details of each state's procedures.
It's worth recalling that state legislatures adopted lethal injection in response to the shortcomings of the electric chair and the gas chamber, which themselves were presented as more humane alternatives to hanging. All of these methods, if they're done properly, can kill a man with minimal pain, but they can also be botched, and none is pleasant to witness.
On that score, it's striking that abolitionists‚ accounts of botched executions often feature gruesome details that do not indicate prolonged suffering, such as involuntary defecation, smoking skin, and accidentally severed heads. If a man is already unconscious or dead, the condition of his body cannot cause him any discomfort. Why does it still upset people who see or hear about it?
This is a question that should interest conservatives who believe disgust reflects moral intuition. If the aim is to quickly and reliably kill people while inflicting as little pain as possible, it would be hard to improve on the guillotine or a close-range shot to the back of the head. Yet we shrink from such methods, perhaps because they too vividly display the reality of killing a man in cold blood.