Fred Clark at Slacktivist has a good discussion of the hanging of Saddam Hussein.
The problem with the death penalty is that it’s awfully final. Even if the victim is known with a certainty to be guilty, death doesn’t just remove them with no hope of recovery, it also extinguishes whatever they might know. And if someday Iraq wants to know what Saddam did, and who helped him, the best source of information is no longer available.