One side cares, as Cohen explains, principally about finality. That's why supporters of Perry (who claims never to have lost a night's sleep over an execution) and Supreme Court Justice Antonin Scalia (who wrote in 2009 that "this Court has never held that the Constitution forbids the execution of a convicted defendant who has had a full and fair trial but is later able to convince a habeas court that he is 'actually' innocent") believe that for the process to work, it must eventually end—and that in order to achieve that end, some error is inevitable. The victims' rights movement, which has had so much influence over the criminal law in this country, uses the same reasoning, except that it's called "closure." The theory here (not always supported by the empirical research) is that victims of violent crime can only heal and move past a tragedy once they have had some formal ending to the legal inquiry that, in states that allow for capital punishment, must take the form of capital punishment.
But the nature of the judicial system is that certainty and finality are always in tension. It's almost impossible to have both. And with the host of DNA exonerations, the well-publicized executions of at least arguably innocent men like Cameron Todd Willingham, the public may come to feel more inclined toward certainty than finality. I think that the kinds of errors supporters of the current system are willing to tolerate are simply not the kinds of errors most Americans can stomach. Most Americans will eventually reject the idea that there can be no check on profoundly entrenched racial bias, overzealous cops, jailhouse snitches, and crappy forensic evidence. Cohen's point, I think, is that those errors are also becoming harder to defend. A groundswell of doubt about the entire capital system was the motivation behind the most compelling slogan of Davis' supporters: "Too Much Doubt."
Advances in science and the empirical research on erroneous convictions are only going to create more doubt in the future. There is an almost unlimited supply of prosecutorial error and misconduct to draw on, and as it grows so will public uncertainty. And as the new media and social media broaden the debate about the death penalty, the folks who are leery of that uncertainty are ever more likely to be heard. America's conversation over capital punishment has long been weighted toward the interests of finality. But there is a growing space for reason and doubt and scientific certainty. It's hardly a surprise that prosecutors, courts, and clemency boards favor finality over certainty. That—after all—is the product they must show at the end of the day. But maybe the surprise, and the faint hope, of the massive outcry over the execution of Troy Davis, is that the rest of us have found a way to demand more from a system that has—for too long—only needed to be good enough.