I don’t know very much about the Troy Davis case. Even given that I’m someone with a tentacle in almost every corner of the internet, it somehow passed me by; these things happen. I have read a number of things both from official news sources and from people whose judgment I trust which allege that the convictions against Davis were based on the flimsiest of evidences, now discredited; I see no reason not to believe these allegations, given my general skepticism towards pretty much any criminal allegations made by agents of the state. The standard is supposed to be "beyond a reasonable doubt," and from what I can tell that standard wasn't met. Of course, on the morning of this writing, it doesn't matter anymore for Davis, who was executed in Georgia last night. But maybe it might matter to others in the future.
There are two tragedies when a death penalty is enacted, and the most obvious and direct one is the possibility (or, indeed, sometimes certainty) that an innocent individual has been irrevocably deleted from society. I take it for granted that executing an innocent person is never acceptable, and have no interest in deploying arguments designed to convince anyone of this. Anything else turns the concept of justice into an absurdity. But the second tragedy of the death penalty, which operates even when it's exercised on the obviously guilty, is the tragedy of what we become as a society when we condone (or worse, demand) the imposition of the ultimate sanction by the state.
There is a good dialogue on the death penalty between Justin Smith and Gerald Dworkin here. One of the notable features of this dialogue is the distinction between Dworkin's relatively abstract philosophizing and Smith's repeated appeals to the idea that the death penalty is incompatible with our stated societal goal of not being cruel. I'm not knocking Dworkin here, but I think it should be obvious where my sympathies lie. We have a certain notion of ourselves as a society capable of mercy, and not only that, but incapable of (or at least strongly averse to) cruelty (this tendency is so strong that even in circumstances where the treatment is obviously cruel, c.f. waterboarding, Bradley Manning's confinement conditions, etc., the overwhelming initial reaction by defenders of that treatment is not to accept the cruelty as necessary but to deny that it's cruel at all). And I side with Smith when he says that we can't reasonably sustain those notions when we allow ourselves to employ the mechanism of the state to take the life of another human being, guilty or not.
Because in the end, I believe that a necessary condition of being a moral society is that we be a society that rejects cruelty and bloodlust, even for the worst among us. And when we allow ourselves to be ruled by that bloodlust, we take one step down the road that leads to a descent into barbarism.