Very interesting article. You do have to read him carefully, though. Look here:
in a country where the federal government and thirty–eight of the states (comprising about 85 percent of the population) believe the death penalty is sometimes just and appropriate?If you don't read that carefully, it would seem as though he's asserting that 85% of the population favors the death penalty.
But in any case, am I correct in stating that the death penalty is justified not because it provides a deterrent, but because it is simply a just punishment in some cases? The proposition that the death penalty provides deterrence is one of the pillars that death penalty supporters use to justify it's continued usage.
Deterrent effect is extremely difficult to measure. Most studies I have seen referenced show some small, perhaps statistically insignificant, deterrent effect. Deterrence is a good thing, but is not the sine qua non of legal validation. Hence, the discussion of the common law and the other bases justifying the penalty.