Should the death penalty be allowed?

Although I agree with your position generally, I feel like your argument is weak. If we say that the reason not to kill other humans is because it is objectively morally incorrect to do so, point blank, we're opening up the possibility for someone simply to reject that shared morality and find no good argument to avoid murder. We need to look at it from a less absolute angle -- there are solid, concrete reasons for not executing our prisoners that have nothing to do with whether or not they "deserve" it (although the concept of someone "deserving" death is an even worse example of an absolute morality pressed where it doesn't belong). The point you bring up about Scandinavian prisons successfully reducing recidivism by actually trying to reforms criminals is a better place to start. If it's possible to prevent most criminal behavior through education, it stands to reason that society as a whole has more to gain by applying that education to all sane criminals, with the .001% that are "irredeemable" being locked up for a longer time -- if nothing else because that's actually cheaper than executing them.