The problem is that many of those many factors and issues may be brought in improperly to cloud the question of whether the ritualistic, cold-blooded killing of a human by a state can ever be justified in principle. Is the death penalty a deterrent, for example? I don't care. That's irrelevant to the moral question. Should we have to pay out big money to keep a murderer incarcerated for decades? If our judgement is made on a moral and principled basis, that also is entirely irrelevant.