This from someone who is theoretically ok with capital punishment, but in practice opposed. I do not trust that the state gets the right people all the time. I also think it's too slow and burocratic a process to work as anything other than revenge.
(1.) In high profile federal cases executions occur relatively swiftly. The reason it's slow in many cases is due to all of the procedural safeguards that we have in place to protect defendants.
(2.) People often dismiss retribution without explaining why it's not a valid basis for punishment. Personally, I believe Norway is morally bankrupt for viewing rehab as the only acceptable goal of punishment, as the Breivik case vividly illustrates. You may not find the Breivik sentence appalling, but you should recognize that you are in the minority. And if this guy is going to lecture Americans about the ethics of punishment, I have a right to criticize the Norwegian approach.
(3.) Alex Kozinski made an interesting observation about the possibility of wrongfully executing someone: due to all of the procedural protections in place and all of the resources devoted to capital cases, the odds of being wrongfully executed are an order of magnitude lower than the odds of being wrongfully dealt a life sentence.
Given how awful life imprisonment is, I totally reject the premise that capital punishment can only be justified if it is infallible.