Invite a philosopher to participate in a discussion of a legal controversy and the conversation will either ascend or descend to a different level, depending on one’s point of view. And if that philosopher also has an intense interest in the history of ideas, including arguments for and against capital punishment, the conversation is bound to veer off into several directions. I will do my best to maintain some measure of focus.
Professor Garrett has given us an excellent overview of the impact of DNA testing on the criminal justice system, especially in capital cases. He correctly points out that the high degree of certainty provided by DNA tests can be invoked by either side of the death penalty controversy. Opponents of the death penalty can point to numerous cases of wrongful convictions in the past, whereas proponents of the death penalty can provide strong assurance that such errors are far less likely to occur in the future, thanks to DNA tests.
The possibility of error in criminal prosecutions, and its implications for capital punishment, have long been discussed by legal philosophers. In An Introduction to the Principles of Morals and Legislation (1780), the utilitarian philosopher Jeremy Bentham pinpointed the death penalty as the most egregious example of a punishment that cannot be corrected in the event of a wrongful conviction:
It may happen that punishment shall have been inflicted, where, according to the intention of the law itself, it ought not to have been inflicted: that is, where the sufferer is innocent of the offence. At the time of the sentence passed he appeared guilty: but since then, accident has brought his innocence to light. This being the case, so much of the destined punishment as he has suffered already, there is no help for. The business is then to free him from as much as is yet to come. But is there any yet to come? There is very little chance of their being any, unless it be so much as consists of chronical punishment: such as imprisonment, banishment, penal labour, and the like…. The most perfectly irremissible of any is capital punishment. For though other punishments cannot, when they are over, be remitted, they may be compensated for; and although the unfortunate victim cannot be put into the same condition, yet possibly means may be found of putting him into as good a condition as he would have been in if he had never suffered. This may in general be done very effectually where the punishment has been no other than pecuniary. 
In 1963 the libertarian Nathaniel Branden expressed a common position on capital punishment. He distinguished “two separate aspects” of the controversy, namely, “the moral and the legal.” Capital punishment, viewed from a moral perspective, is fully justified. The man “who commits willful murder, in the absence of any extenuating circumstances, deserves to die.” This position is not a matter of revenge or deterrence but is “the logical and just consequence of his own act,” an “expression of the moral principle that no man may violate the rights of another with impunity….”
The legal question of capital punishment “is of a different order” than the moral question, according to Branden.
If it were possible to be fully and irrevocably certain, beyond any possibility of error, that a man were guilty, then capital punishment for murder would be appropriate and just. But men are not infallible; juries make mistakes; that is the problem….It is the possibility of executing an innocent man that raises doubts about the legal advisability of capital punishment. It is preferable to sentence ten murderers to life imprisonment, rather than sentence one innocent man to death….The problem involved is that of establishing criteria of proof so rationally stringent as to forbid the possibility of convicting an innocent man. 
We may presume that DNA testing meets the high standard of certainty required by both Bentham and Branden, but even if this is the case it would not lead those men to endorse the same conclusion. Branden, a natural-rights philosopher, endorsed capital punishment, in theory, on moral grounds, using an approach to punishment known as retribution to argue that a murderer deserves to die. Bentham, who rejected natural rights in favor of a utilitarian standard, opposed the death penalty, primarily because he viewed it as a less effective deterrent than life imprisonment, especially if hard labor is involved.
Nathaniel Branden’s moral stance on capital punishment is similar to that of Immanuel Kant. In The Metaphysical Elements of Justice (1797), Kant argued that an accused criminal “must first be found to be deserving of punishment before any consideration is given to the utility of this punishment, for himself or for his fellow citizens.” The appropriate standard of punishment is “the principle of equality,” as “illustrated by the pointer on the scales of justice.” Undeserved evils that a criminal inflicts on another person should be regarded, in effect, as perpetrated on himself. If I strike another person, I strike myself; if I rob from another person, I rob from myself; if I kill another person, I kill myself. It is only this conception of juridical equality that can determine the quality and the quantity of a just punishment.
Thus, according to Kant, “Anyone who is a murderer – that is, has committed murder, commanded one, or taken part in one – must suffer death.” This punishment is a legal duty demanded by the principles of justice, not a legal option to be decided at the discretion of a judge. 
The opposing view on capital punishment, such as that taken by Jeremy Bentham and other utilitarians, relied heavily on the arguments of Cesare Beccaria, the leading philosopher of the Italian Enlightenment and author of the immensely influential Essay on Crimes and Punishments (1764). Beccaria combined a natural rights approach with a utilitarian analysis – a common hybrid during the eighteenth century, until Bentham drove a wedge between the two positions – so Beccaria criticized the death penalty on two grounds.
Beccaria’s brief moral case against capital punishment invokes the notion of a social contract to argue that we would never voluntarily agree to be killed by a government that has a duty to protect our rights. Beccaria wrote:
What manner of right can men attribute to themselves to slaughter their fellow beings? Certainly not that from which sovereignty and the laws derive. These are nothing but the sum of the least portions of the private liberty of each person; they represent the general will, which is the aggregate of particular wills. Was there ever a man who can have wished to leave to other men the choice of killing him? Is it conceivable that the least sacrifice of each person’s liberty should include sacrifice of the greatest of all goods, life? 
Kant, who also embraced a version of social contract theory, rudely dismissed this argument as “sophistry” and countered with a succinct and persuasive rebuttal. But Beccaria’s moral case against capital punishment, which I quoted almost in its entirety, was but a small part of his argument. Beccaria’s utilitarian case against capital punishment was more extensive and far more influential, having been used by Voltaire, Bentham, and other legal reformers.
According to Beccaria, punishment has two fundamental objectives: to restrain the criminal from committing additional crimes and to deter other members of society from committing the same crime. The first purpose is served by imprisonment, so we are left with the issue of deterrence. Beccaria’s discussion of deterrence sparked a long and complex debate that continues to this day. Here is a summary of his argument, according to which life imprisonment (with hard labor) is a more effective deterrent than the death penalty:
It is not the intensity of punishment that has the greatest effect on the human spirit, but its duration, for our sensibility is more easily and more permanently affected by slight but repeated impressions than by a powerful but momentary action….It is not the terrible yet momentary spectacle of the death of a wretch, but the long and painful example of a man deprived of liberty, who having become a beast of burden, recompenses with his labors the society he has offended, which is the strongest curb against crimes. That efficacious idea – efficacious, because very often repeated to ourselves – “I myself shall be reduced to so long a miserable condition if I commit a similar misdeed” is far more potent than the idea of death, which men envision always at an obscure distance. 
More is involved in Beccaria’s argument than may be immediately apparent, and Jeremy Bentham elaborated upon its implications in (some would say) excruciating detail. Fortunately perhaps, I can plead space limitations and stop here, leaving any additional comments to later in our discussion.
For now, suffice it to say that I think a compelling moral case can be made against capital punishment, so I do not accept the retributive theory of punishment defended by Kant and many other philosophers. Like most critics of capital punishment, I regard the problem of legal certainty, which has supposedly been solved by DNA testing, as little more than a footnote to a much broader and more fundamental moral controversy.
 The Works of Jeremy Bentham, published under the Superintendence of his Executor, John Bowring (Edinburgh: William Tait, 1838-1843). 11 vols. Vol. 1. Chapter: CHAPTER XVII.: OF THE PROPERTIES TO BE GIVEN TO A LOT OF PUNISHMENT.
 The Objectivist Newsletter, Vol. 2, No. 1 (January 1963), p. 3.
 Immanuel Kant, The Metaphysical Elements of Justice, trans. John Ladd (Bobbs-Merrill, 1965), pp. 100-104.
 Beccaria, On Crimes and Punishments, trans. Henry Paolucci, in The Enlightenment: A Comprehensive Anthology, ed., Peter Gay (Simon & Schuster, 1973), p. 732.
 Ibid., p. 733.