Deterrence and the Death Penalty

In posting a thoughtful op-ed by Maryland's governor on the morality of the death penalty, Rick suggests that the governor's analysis seems to have been informed by Catholic teaching.  That may be right, but one central feature of the analysis struck me as quite un-Catholic: Governor O'Malley appears to place a lot of weight (as Rick notes) on the deterrent effect of the death penalty as an indication of its moral status.  I'm not any sort of expert on this issue, but I would think that questions of deterrence are irrelevant to the Church's moral analysis of the death penalty because deterrence-based justifications adopt an instrumentalist view of human life -- i.e., we can kill him because that will convince other people not to kill.  Am I wrong -- is deterrence a relevant consideration?