If in fact that person was guilty of the crimes of which they are accused then yes, there could be times where the death penalty is justified. The Nuremburg Trials are a good example. However, there are many cases where the death penalty has been handed down to people later proven innocent. Unfortunately this proof often comes after the person has been executed. I agree that the amount of innocent people for whom the death penalty is considered is small, but the death penalty as a policy is simply not worth becoming the perpetrator of the same crimes as those who are being punished. Murder is murder, regardless of whether or not the state commits it, and the death of an innocent prisoner, while rare, is more important to me than the availability of that punishment and the possible cost savings of removing someone from life imprisonment.
And maybe if there were a push for severe limitation of the death penalty, people would support that, but the national discussion is split between leaving it alone or abolishing it. yellowoftops comment is an argument for the latter--if we refuse to change it, we shouldn't use it at all.