Thom Brooks has posted a short paper laying out a retributivist argument for rejecting the retributivist justification of the death penalty. His argument is epistemological; it does not rely on the notion that capital punishment is not proportionate to murder or that it constitutes cruel or unusual punishment. Here's an excerpt:
A determination of punishable desert may be substantially a determination concerning the state of mind of another. We cannot read the minds of others, but we can bear witness to their actions against an explanatory narrative that may give us some insight into why another did what they did. The claim here is not that we can never grasp guilt, but that our certainty is always in doubt. This does not mean that retributivism cannot justify punishment because it can offer us ‘no perfectly just judgements’. However, it would mean that capital punishment is especially problematic. When we impose capital punishment on a convicted murderer, there cannot be room for error as the murderer can never be brought back to life afterwards. If there remains a substantial risk of error demonstrated by advances in scientific testing in cases where a person has been sentenced in a fair trial beyond reasonable doubt, then we have good reason on retributivist grounds to reject capital punishment in favour of an alternative sanction.