Elden has a nice rundown of the issues involved. I think Dahlia Lithwick notes well that there are two sides talking past one another: those who are horrified not just at judicial killing but also at the idea that someone who may be innocent is given the death penalty (and thus see the other side, perhaps, as adhering to too few strands of supposed evidence to justify the execution), and those who
care, as [Andrew] Cohen explains, principally about finality. That’s why supporters of [Texas Governor Rick] 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.
Read that quote again from Scalia–lovely way he outdoes the worst so-called postmodernist with his scare quotes around “actually”–which I think outlines well how sovereign fiat is its own end and must remain unquestioned, even when what is at stake is so grave and the judgment itself is so questionable.