Capital punishment has recently become an increased focus of international attention and debate. From an ethical perspective, many of the arguments for and against the death penalty are missing a consideration of key issues.
Criminologists consider that the major reasons for criminal penalties are rehabilitation (reforming the prisoner to be a better citizen), incapacitation (preventing the prisoner from committing other crimes), deterrence (discouraging the prisoner and others from further crime) and retribution (society punishing the prisoner as vengeance for a criminal act). Rehabilitation and incapacitation can be achieved through appropriately lengthy jail sentences. The only reasons that could possibly be offered in support of a death penalty are deterrence and retribution.
However, the large majority of experts consider that there is no credible scientific evidence supporting the contention that the death penalty deters criminal behaviour. This is a surprising result for some, but perhaps the criminal mind doesn't think of consequences or has difficulty computing the risk profiles associated with undertaking a criminal act.
That leaves retribution. Consider whether people would advocate retribution in a hypothetical situation. Imagine that you are the world's best neurosurgeon and you have surgically removed a patient's large brain tumour, which would have caused extremely violent outbursts. At the same time, a DNA test links your patient to the violent deaths and rapes of your closest friends. As the world's foremost surgeon, you know that this person will no longer commit such crimes (they were a model citizen until the tumour developed). There is no need for incapacitation, your patient has been rehabilitated through your surgery, and there is no need for deterrence (as people don't plan to have brain tumours). Yet some people might still consider that retribution, through capital punishment, is desirable, despite its unjustness. What would we want to happen if, instead, each of us were the patient?
This hypothetical situation is not unrealistic, because many people and cultures consider it acceptable to kill people against their will. Many religions teach that their gods or deities of choice have killed many others, through great floods, the Passover (death of newborn infants), and much general smiting done without presumptions of innocence and trials. Many people believe that these religious teachings are good. Over time, these beliefs have manifested themselves in many legal systems. Indeed, the four most populous countries, China, India, the United States of America and Indonesia, have the death penalty on their statute books.
Although many might support retribution as an argument for the death penalty, it doesn't conform to modern notions of how we should treat fellow humans. A better alternative to many people's eye for an eye system of morality is one based on an ethical principle that it is wrong to kill other people against their will. There are some exceptions to this of course, self defence being the most notable. According to this principle the death penalty would be forbidden. If it is wrong for one individual to kill another then it should be unacceptable for the state to cause a person's death in civilised societies. The state, as a collective of individuals, should not generally have moral rights that individuals do not have.
Modern societies recognise that prisoners should be treated humanely, consistent with human rights obligations. Some criminals do commit horrific crimes, but capital punishment, torture, or mistreatment of prisoners serves no utilitarian purpose and signals, wrongly, that violence can solve problems. An eye for an eye society is one that is of years gone by, and unsuited to a modern civilised society.
It seems clear then that any countries that want to take the moral high ground and campaign against capital punishment for their nationals who have committed crimes in other countries should abide by some rules.
First, they should not have the death penalty on their own statute books. That would be hypocritical.
Second, they must not consider that killing some people is acceptable according to some of their belief systems. It is hypocritical to denounce killing in somebody else's moral or legal system, if you accept it in your own.
Third, they should not campaign against capital punishment only for their citizens. In moral matters, what is right for one nationality ought to be right for people of other nationalities. To do otherwise is self-serving, nationalistic, and a form of racial/cultural/ethnic discrimination.
Fourth, they should make representations against the death penalty with equal vigour to all countries that have capital punishment. To make representations to one country, and not for example, to China or the United States, indicates bias. International relations are complex, but moral campaigns aimed at one country over others cannot be morally sound.
If countries follow these rules, they can work diplomatically and cooperatively with each other to endorse and uphold the principle that killing others against their will is wrong, and in so doing eliminate capital punishment. This principle should be applied to all people, in all countries, at all times.