I believe there is a difference between ME killing the perp to stop the perp's immediate behavior, and a state policy of killing the perp as a matter of state law. If I see a perp raping a six year-old, the chances of me making a bad ID are slim. If the state is prosecuting a circumstantial case, then the chances for error are greatly multiplied.
Capital punishment in Oregon has been hotly disputed for decades (as in most other States), and after living here for 40 years, I've had a chance to observe the behavior of locally prominent lawyers, politicians, and activists. There have been some few cases where these anti-CPers have subsequently been harmed by violent crime, and it struck me how quickly they converted to supporting CP (I have no names in mind, just a clear memory of "yes, it happened").
Makes a difference when a person lives in an ivory tower. The street teaches the hard lessons of real life.
Do I believe that I would seek vengeance upon someone who killed a member of my family?
Yes.
Isn't CP vengeance of a sort, except monitored by the state to ensure that emotional relatives don't go overboard in taking the law into their own hands, in a sense? I think that in ages past, vengeance has had a tendency to get out of hand when it was taken by relatives.
Family members of victims of murders cannot legally seek vengeance. They demand that the state give it to them thru CP. For them, it means justice. Perhaps that is why it persists.
Self defence, revenge and justice are three differnet things.
I would kill to defend, have the desire to kill for revenge that I might have opportunity to act on, but I understand justice to be different from revenge.