Roberts isn't alone here. Scalia once wrote in a dissenting opinion in a capital murder case that it doesn't really matter whether one committed a crime, only that one was fairly tried. From the dissent in this case:The primary law enforcer in the western world just implied that the law wouldn't allow him to make the right decision; he had to make the legal one.
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. Quite to the contrary, we have repeatedly left that question unresolved, while expressing considerable doubt that any claim based on alleged “actual innocence” is constitutionally cognizable.