“If I think of the kind of a hypothetical where you have an inmate who is about to be executed and knows that this execution may involve excruciating suffering, that inmate requests the involvement of a physician, because he knows that the physician can prevent that suffering from occurring, and if there is a physician who is willing to do that, and we know from surveys that many are, I honestly can’t think of any principle of medical ethics that would say that that is an unethical thing for the physician to do.”