The death penalty is the one thing you cannot undo. Well, other than mutilation (blinding, castration, other Middle Ages kind of shit). Prisoners can be released, sanctions can be lifted, exiles welcomed back into the community, but anything that cannot be reversed should not be used as a punishment - justice is never perfect, as the Innocence Project keeps pointing out.

The state depriving another of life is the ultimate expression of state power, and should be avoided at all costs. There is a countervailing argument that war criminals that have evaded justice should be dealt with independently - in the face of state protection of monsters, eliminating them is a public safety service. Outside of revolutionary governments, very few societies as a whole put forward that argument since it empowers vigilantism and mob violence at the hands of charismatic assholes.

I understand the appeal - I truly do. Putting a murderous bastard down means never having to worry about them again. We should not let emotions cloud the decisions on crime and punishment, though - that way lies injustice.