Let me make it resoundingly clear, I am opposed to the death penalty and I believe that in general the Church is opposed to the death penalty, although certainly not to the point of total exclusion.
That said, I find the concerns expressed in events like these to be totally out of proportion. We're told that the person subjected to this "suffered unduly," and yet what did his victim suffer? This disproportionate concern for the suffering of the guilty seems misplaced. Yes, it is terrible that he suffered. It is even more terrible that he took it upon himself to make another suffer and die as well. Is it possible to suffer unduly under such circumstances? Is it not just possible that he is working out a bit of purgatory on Earth (assuming of course that he sought and found God's grace). I think this focus on the guilty tends to suggest that what he did to another is not so very bad after all.
I oppose the death penalty, but not for reasons of clemency toward the guilty so much as for reasons of the dignity of the individual and other reasons that I think the Church is careful to delineate. And I wonder, are a few moments, or even minutes, of discomfort more suffering that thirty or forty years of slowly deteriorating in a prison? I don't know. But if we want to talk about undue suffering, perhaps the whole picture should be put into perspective.