How many times have you heard it said (see here, for example) that the United States is the only civilized (industrial, first- or second-world) nation with the death penalty? I have never understood the import of this statement. Either capital punishment is justified or it's not. If it is, then we should pay no attention to the fact that other nations disallow it. They are mistaken and misguided. If it isn't, then we should disallow it for that reason, not because other nations disallow it. Either way, it's irrelevant what other nations think, say, or do.
There are only two reasons I can think of for paying attention to what other nations do. The first is that uniformity is inherently good. But this sweeps far too broadly. I don't hear the people who make the claim about capital punishment say that we should adopt the diets, sports, sexual practices, mannerisms, and laws of other nations. How could we? There is no uniformity in these areas. The second is that the other nations may be on to something with respect to capital punishment. Perhaps we can learn from them. But this goes back to what I said earlier. We must ultimately decide for ourselves whether capital punishment is justified. This requires examining all the reasons for and against it. That most or all other nations have concluded that it's unjustified doesn't establish that it is unjustified.
The United States has always been a special place, morally speaking. We have a written constitution. We have a Bill of Rights. We assign a high value to the individual. What critics of the death penalty don't understand is that it is precisely because of the high value we assign to each individual that we execute convicted murderers. (For attempts to explain American exceptionalism that entirely miss this point, see here and here.) The value of a thing is expressed in its cost. The cost of taking innocent human life is death.
Don't dismiss this as sloganeering. The two greatest liberals in modern history, Immanuel Kant (1724-1804) and John Stuart Mill (1806-1873), a deontologist and a consequentialist, a retributivist and a utilitarian, a German and an Englishman, a rationalist and an empiricist, defended capital punishment. I'm not suggesting that we should allow it because they supported it. That would be as fallacious as the argument that we should disallow it because Europeans reject it. I'm suggesting that the reason they supported it is their firm belief in the worth, dignity, and inviolability of the individual.