I think the main reason to end the death penalty is different: capital punishment is a very dangerous power for any nation to have over its inhabitants. Many of us are accustomed to living in stable democracies. Yet how easily and quickly could any of them become a totalitarian state or a reign of terror, where innocent people (especially activists and dissidents) are executed on false charges? Contemporary nations should not have the death penalty. If any nation legalizes it, that should serve as a canary to other nations that said nation is about to become an oppressive state with little appreciation for human rights.

Just look at the death penalty historically, even not too long ago, and how it was used in many now "civilized" countries to kill people for minor crimes. Or look at the contemporary world, at the countries that use capital punishment and the crimes they prescribe it for (even assuming they were prescribing it justly.) Aren't these countries themselves a good argument that it's best to err on the side of not having capital punishment at all, anywhere?

I think some criminals are truly awful and deserve to die for their heinous crimes. Yet it's simply too dangerous for any society to have the death penalty, even for these heinous crimes. As an intellectual, I'd be somewhat or very scared of living in any of the contemporary countries that have capital punishment (not to mention the ones that essentially allow street executions by the police. But that is another topic for another day...)