But that shouldn't affect how we think about such issues. Political tactics are not a substitute for political principles, and yet people confuse the two all the time. The death penalty is always more popular when crime rates are high. We think that if Murderer A is executed for killing someone else's child, that will make it much less likely that Murderer B or C will murder our own child. When murder rates are low, support for the death penalty decreases because people are less afraid.
This utilitarian calculus is not only understandable but rational and deeply seductive. Death penalty opponents understand this, which is why they insist that deterrence has no effect. I think this is poppycock, the studies saying otherwise be damned. It defies common sense to think that Chinese officials won't be deterred at all by Zheng's demise. At minimum, this will raise the price of bribes in China - which, as any economist will tell you, means that at the margins there will be fewer bribes. That the statistical evidence in the U.S. allegedly doesn't support the deterrence argument is more of a commentary on the inefficiencies of our criminal justice system.
But the point is that it shouldn't matter whether capital punishment is a deterrent. The death penalty cannot be justified by the deterrence argument alone. As the late sociologist Ernest van den Haag wrote, "Deterring the crimes, not yet committed, of others does not morally justify execution of any convict (except to utilitarians, who think usefulness is a moral justification)." It is child's play to make the utilitarian case for executing shoplifters, but as all but the most morally stunted should see, hanging one shoplifter cannot be justified by the argument that it will deter another.
Like van den Haag, I support the death penalty because I believe that in some cases the death penalty is just. But, save perhaps in the realm of military justice or some truly grave crisis, executing to set an example for others is an indefensible rationalization of mob rule. That is what they have in China and, too often, that is what some advocates of the death penalty argue for here.