Thing is, there is overwhelming evidence that the death penalty does not act as a deterrent in real life. I mean, you are putting forward an interesting ethical thought experiment, but the fundamental parameters for this to work only exist within a fictive model. There's nothing wrong with that, but it's easy to then make the mistake and transfer the conclusions we take from the model into reality, and base decisions made in real life on these conclusions.
Btw this one of my main points of criticism of neoliberal economics, its conclusions are not wrong in themselves, but they only work inside a very specific model, where things such as perfect market information or perfect competition exist. In reality, where these things do not exist, the results drawn from the model do not always lead to the intended results. I realise that the simplification of reality is a basic problem, or indeed the fundamental definition of models, but it really becomes worrysome when the differences between models and real life are so fundamental, that they lead to completely different results, but this is not fully realised or taken into account.
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I fully agree that we should strive for perfection even though it is unattainable. But the important thing is that we really have to take into account that perfection is unattainable, and include that in our concepts and systems.