The death sentence is interesting to consider from a Popperian standpoint. A couple of things about it stand out to me:

1. It's anti-fallibilist because it’s irreversible. It’s final.

   Popperians prefer to make piecemeal, reversible changes in life. They try to avoid any course of action that is irreversible. That is not always possible, but it's worth trying. Fallibility alone is enough to see why: any course of action can be a mistake, and you want the ability to correct that mistake. One way is to undo it, but the death penalty cannot be undone. Once executed, it is forever entrenched. Killing someone for a crime they did not commit is perhaps the ultimate mistake.

   Preserving the means of error correction and working against the entrenchment of errors is a prime Popperian objective. David Deutsch has elaborated on this in his book The Beginning of Infinity (particularly in chapter 10).

   To be sure, wrongfully incarcerating someone instead of killing them is still awful – some might prefer death over being imprisoned for decades. It can't be 'undone' in the sense that you could give back the years that have been robbed from that person. But at least you can set them free. They're still alive; they can still change their life for the better. All is not lost.

2. The death sentence is pessimistic because it considers the sentenced person to be irredeemable.

   People can be persuaded. It can be extraordinarily difficult, but given how the mind works it is always possible to change it. This means, in turn, that even people like Ted Bundy or Charles Manson can change for the better. Given the right knowledge, their mistaken ideas can be corrected.

The reason the vast majority of people do not commit crimes is not that they're afraid of the punishment. Instead, they are convinced that committing such crimes would be wrong. They are decent people overall, guided by moral principles. Indeed, punishment may have an effect opposite to the intended one: it may turn its victim stubborn and more convinced that his actions are right and everyone else wrong. It is only natural to want to resist force. Consider a child who gets grounded: maybe he made a genuine mistake, however, imprisoning him in his room will not only fail to correct it but may well entrench it. I expect it's similar for adults in prison: if they manage to correct their mistaken ideas, it's not because of punishment, but in spite of it.

Maybe punishment is the result of the age-old epistemological misconception that people can be made to change behavior like animals: through positive and negative reinforcement. That's just not how people work. I suspect we have inherited this mistake from our forefathers, who didn't know any better.

That is not to say that I'm advocating pacifism – I strongly oppose it – or that I see no room at all for punishment in our society. Unfortunately, we do not yet know how to live without it. And, faced with the threat of a monster like a serial killer, be he redeemable or not, you need to protect yourself and your loved ones. If someone intent on causing you harm breaks into your home at night, that is not the time to sit down together and have a nuanced discussion to try to change his mind – it's a shoot-first-ask-questions-later kind of situation. But one day we may well live in a society that has enough knowledge and wealth, whose institutions have adapted, and whose members are so good at persuasion, that no punishment is necessary. There is no limit to how persuasive we can become.

Until then, when faced with a bully, we have no choice but to hire the biggest bully – the state – for protection. Punishment is a sign of incompetence, and the death sentence represents ultimate incompetence. I've quoted William Godwin previously:

If he who employs coercion against me could mould me to his purposes by argument, no doubt he would. He pretends to punish me because his argument is strong; but he really punishes me because his argument is weak.

– William Godwin (1798). “Enquiry Concerning Political Justice, and Its Influence on Morals and Happiness: By William Godwin”, p.337

Thanks to Amaro Koberle for reading a draft of this post.