Edward Feser has written a short response to Christopher Tollefsen, who argues that capital punishment is intrinsically immoral.  Feser does a good job showing that if one believes in the principle of proportionality, that capital punishment is moral at least in principle, even if we might haggle over when we should apply it.  I particularly liked the first part of the article because Feser laid out a nice, succinct case for the notion of retributive punishment.  In my experience, those most opposed to capital punishment are opposed because they see punishment as being primarily corrective in nature, or for the purpose of quarantining evil, not for retribution.  This is a deficient view of punishment, and leads one to view capital punishment as either unnecessary or immoral.