Retributivism has been a tough nut to crack for those interested in promoting the need to end capital punishment. Other theories of punishment are more straightforward: you need only convince deterrence-proponents that the death penalty does not deter or convince others it does not helpfully rehabilitate or provide for 'restoration', etc. But what to do if you accept the general premises of retributivism? If a person has desert in relation to his (moral) responsibility for murder, then how might you convince a retributivist that s/he should oppose capital punishment even if you held further that death could be 'in proportion'?
There have been past attempts at this issue and the general strategy -- used by Daniel McDermott and Stephen Nathanson -- is to argue that to deserve punishment is to be subject to a just distribution. Thus, if there is disparities in the just distribution of capital punishment, then it should be abolished. My counter objection is that this is not a retributivist argument, but an argument about distribution (and not desert). Retributivists could counter that perhaps there is racial disparity in distribution, but then we need to end the disparity: one way might be to end capital punishment, but another might be to use it more consistently (and perhaps more often). If you believed death was a proportionate punishment, then you might argue we should work more to ensure fair distribution to all deserving.
I have tried to argue for a new view in my published work, such as "Retributivist Arguments against Capital Punishment" (in Journal of Social Philosophy) and "Retributivism and Capital Punishment" (in Retributivism (OUP, 2011)). My argument centres on desert-based reasons for why a retributivist should oppose the death penalty.
I note the background because I've become alerted to a few excellent blog posts examining my positions on capital punishment that can be found here and here that I recommend for readers with interests in this subject area. I welcome all comments on the matter -- and hope that readers will be convinced by my arguments, but I'm very open to refining my position further!