edit: I heard of countless cases of young black men being incarcerated for murder in Texas in the 60's with police going quite out of their way to obstruct the justice process.
Pragmatically, I think capital punishment is a good thing to have since it causes us to give the criminal justice system an extra degree of scrutiny. I'd rather be falsely convicted of a capital crime than a life in prison crime. I lose my life either way, but more people are motivated to exonerate me if I'm facing the death penalty than for life in prison.
And they still have another 20 to live. And they get millions of dollars in restitution. So, an innocent man is executed - possibly the worst injustice imaginable. Who get's to tell his children, his parents?
That's an interesting second point, a bit of a gamble though.
Suppose you were wrongly imprisoned for 20 years, but later freed. You could reasonably argue that you would rather have been killed (but would you?). You cannot reasonably argue that I, or your friend, or your mother would prefer to be executed rather than to live in prison for 20 years.
Second, what does the extra scrutiny matter if we still execute innocent people? If there were no capital punishment, we by definition would not execute any innocent people.
Your no-right-to-life argument is predicated on your own stated, but not proven, value judgement, but it's murderous to force this on others.
Extra scrutiny matters because the goal is not minimizing execution of the innocent, but minimizing punishment of the innocent. I don't see how wrongly making a person live out their life in jail is significantly worse than killing them. Extra scrutiny matters because it would reduce our error rate. I'd prefer one mistaken execution to two mistaken life imprisonments.
Note that this latter point is strictly a utilitarian one, not a moral one. In a world without assorted lawyers/activists/etc working to prevent wrongful executions, the argument would not apply.
And it is relatively rare that an innocent man is convicted and sentenced to death. We can have a system that forces all of us (as taxpayers) to pay a HUGE amount of money keeping all of the vile criminals alive for a chance that they're in the <1% innocent who were wrongly convicted. Does that make good financial sense though?
Also, I'm talking about the future, not the past. Sure there were many wrongful convictions in the past, but the same technology that is freeing those who were convicted wrongfully in the past is the same technology that will help prevent innocents from being put on death row in the future.
I agree that a lot of cases got turned on their head when DNA matching became available, but what technologies do you envision providing the same sort of disruption in the future?
I definitely think that most countries need to think about the crimes that they imprison people for. Imprisonment is super expensive partly because it's used so often. If only serious criminals went to prison, the social impact of imprisoning someone would be much greater and the overall cost would be lower...
There's good reason to believe that lie detectors might work much better in the laboratory, on test subjects telling inconsequential lies to experimenters and knowing they'll go home, than on real suspected criminals telling vitally important lies (or truths) for life-or-death stakes.