William Saletan complains in Slate that death penalty opponents like to object to certain aspects or instances of execution, when they really oppose execution altogether. I'll go Saletan one further: even those who argue against capital punishment in general are really arguing, more broadly, against punishment itself.
There exists, to be sure, one (single) argument against capital punishment: because it is completed instantaneously, it allows less time for evidence of innocence to be gathered, and is therefore somewhat more likely to be exacted in its entirety upon an innocent than, say, imprisonment for life. I don't personally find that argument convincing, but those who do at least have a case to make.
All other anti-execution arguments, though, are in fact simply arguments against punishment tout court that happen to gain extra force when invoked against the death penalty, by virtue of the fact that death is the most serious punishment currently under consideration in our society. Suppose, for instance, that capital punishment were to be abolished tomorrow; is it reasonable to assume that life terms of imprisonment would then be handed out with complete racial equity, sparing the mentally deficient and the rehabilitateable, and never imposed upon the innocent, or in a spirit of revenge, or upon kidnappers (thus reducing society, some would say, to the criminal's level)? On the contrary, all these familiar arguments against capital punishment can be applied equally to the case of life imprisonment--and would be, but for the severity of the death penalty, which makes a life term seem lenient by comparison.
In fact, we don't need to guess at this outcome; we can observe it empirically. When Canada abolished the death penalty in 1979, it was replaced with a maximum punishment of 25 years to life (that is, life imprisonment with no parole permitted for at least 25 years). The abolition was considered quite controversial, however, until a second parliamentary vote confirmed it in 1987. Almost immediately after that second vote, several prominent Canadians began arguing that 25 years was too long a minimum punishment for murderers who had demonstrated that they had been rehabilitated; not long thereafter, a Supreme Court decision created a murderer's "right" to a special hearing after fifteen years in jail, to determine parole eligibility. No such right had ever even been suggested, of course, until after the abolition of the death penalty had been assured. Only when the 25-year minimum became the "ultimate" penalty did its severity come under scrutiny--and unsurprisingly, it was found by many to be too severe.
Now, I actually sympathize quite strongly with those who feel a deep unease about punishment. Punishment is an ugly, painful, repulsive duty, and I would be the first to abandon it entirely if it were remotely feasible to do so. It gives disturbing expression to human moral imperfection--an imperfection that all of us, punishers and punished alike, share. It confronts us with the worst depths of our own flawed humanity, and rudely reminds the most honest among us that we, too, may require it, or the threat of it, to keep us from giving free rein to our evil impulses. It would be a much better world if we could abandon punishments altogether, starting with the most severe and continuing down to the mildest. But, alas, we cannot, and those who agitate to start that process, thinking it ennobling to do so, are living in a seductive but dangerous fantasy world.