Responding to Gil's comments:

I don't see any support in it for the assertion that we might want to keep someone alive indefinitely just because his crime was so evil. You have only described the value of extracting certain specialized knowledge that he might have. This is not the same thing (although there is some psychological and historical value that might corrolate with this).

Well, if the psychological and historical value might correlate with the degree of evil, then it can happen that it is right to keep a criminal alive because his crime was so evil, can't it?

But, surely, this is only a reason to keep him alive temporarily.

How long is temporarily? A criminal might reveal decades years later that he had committed additional crimes (thus exonerating someone else who had been under suspicion). A researcher might want to interview all murderers in a given category, to test the theory that a certain type of childhood experience predisposes a person to murder. A historian might want to interview a tyrant decades later to test a startling new theory about the events in which the tyrant too part.

We want to treat people fairly, not equally. If one person deserves execution, then another person's treatment should be irrelevant.

No it shouldn't. If a penal system gives incentives to evil people to commit worse crimes than they otherwise would, that is an undesirable property.

I expected The World's reason to avoid executions to have something to do with our fallibility and the moral horror of executing the innocent.

Well, the title is "Further Thoughts on the Death Penalty", not "The Complete Case Against the Death Penalty".