At first glance, the death penalty seems cruel, unusual, horrendous, and uncivilized. It is one thing, the argument goes, for a murderer to bump someone off; this is truly an abomination, since all of human life is precious. However, it is quite another, and far worse, for society as a whole to kill such a person in response, retaliation or revenge, for we, at least, if not the criminal, are supposed to be enlightened. According to the popular bumper sticker: "Why kill people who kill people to show that killing people is wrong?" Then, as a purely pragmatic issue, it costs more to fry an inmate on death row (due mainly to legal costs) than it does to imprison him for life, and such a penalty has little or no disincentive effect in reducing the murder rate.
What the murderer has done, essentially, to his victim is, in effect, steal his life away. If there were but a machine that could transfer the life out of the dead victim and into the live murderer (I am inspired in this fanciful example by Robert Nozick’s Anarchy, State and Utopia, must reading for all non-libertarians) it would be the paradigm case of justice to force him into this machine, and make him disgorge the life he had stolen. It would be a matter of supreme injustice to refuse to do so. Who knows? Maybe in 500 years (if we don’t blow ourselves up before that time) such a machine will actually be created. It doesn’t matter. By use of this example, we can demonstrate that the murderer’s life is forfeit now, for justice is timeless.
If the murderer is not the legitimate owner of his own life in the future, or even hypothetically, he is not now either. The point is, to reply to the bumper sticker mentality of some commentators, it is not necessarily wrong to kill people. It is not impermissible in self-defense, nor is it to kill those who no longer have entitlement to their own lives. Let the message go out, loud and clear: if you murder, you give up the right to your own life. (I am assuming arguendo that innocent people are not executed for murder; given the congenital inefficiency of government operation, this is the only legitimate reason to oppose the death penalty.)
Of course, we do not have any such machine at present. To whom, then, does the murderer owe his life? Obviously, to the heirs of the victim. If I murder a family man, for example, his widow and children then come to "own" me. They can put me to death, publicly, and charge admission for this event, or they can force me to do hard labor for the rest of my miserable life, the proceeds to go to them. It is a crime and a disgrace that such criminals now enjoy air conditioning, television, exercise rooms, etc. They owe a debt to (the heirs of) their victims, who are now, to add insult to injury, forced to pay again, through taxes, to maintain these miscreants in a relatively luxurious life, compared to what they richly deserve.
As for the pragmatic argument, it is simply silly. Yes, economists who ought to know better have found no statistically significant correlation between reducing the murder rate and being or becoming a death penalty state. But that is only because murderers, like most of the rest of us, pay attention not to dead letter laws, but to actual penalties. (It is fallacious to regard murderers as irrational: very few conduct their business in police stations.) When multiple regressions are run on murder rates, not against death penalty status, but with regard to actual executions, the evidence is consistent with the notion that such punishments reduce these crimes. (Isaac Ehrlich has done yeoman work on this issue.) This is entirely compatible with the economic principle of downward sloping demand: the higher the price, the less people wish to access. This holds for all human endeavor: cars, pizza, and, yes, murder too. Nor is it possible not to regard murder as a stiffer penalty than life in prison. Were this not so, we would scarcely find the denizens of death row trying desperately to stave off, or better yet overturn, their executions.
As for the costliness of executions, this is entirely a function of present judicial functioning, which can be changed with the stroke of a pen.