In fairness to Justice Scalia — and despite the fact that I don't think he's usually fair to others — I should clarify/correct something. There's no record of him actually saying that it was inarguable that Henry Lee "Buddy" McCollum was guilty. He presumed McCollum was guilty when he brought him up but so did Justice Blackmun, who was arguing against the Death Penalty for McCollum in that case.
Blackmun actually had two arguments and neither one was that McCollum hadn't done what they said he'd done. One was, and I quote —
Buddy McCollum is mentally retarded. He has an IQ between 60 and 69 and the mental age of a 9-year old. He reads on a second grade level. This factor alone persuades me that the death penalty in his case is unconstitutional.
The other argument was that McCollum's three convicted cohorts in the rape/murder were not sentenced to death; just the guy with the mental age of a 9-year old. Scalia cited the brutal crime to say that death-by-lethal injection seemed "enviable" compared to the crime that had been committed. (Apparently in Scalia's world, the way in which we execute murderers doesn't have to be humane. It just has to not be as bad as what they did to their victims.)
Elsewhere though, Scalia has been a firm believer that innocent people are never put to death by our government. In 2006, he wrote, "If such an event [an innocent person's execution] had occurred in recent years, we would not have to hunt for it; the innocent's name would be shouted from the rooftops by the abolition lobby."
The flaw in that argument, I've always thought, is two-fold. One is that once someone is put to death, it is rare that their innocence is ever investigated. The state that executed them sure doesn't want to see that proven and often puts great obstacles in the way of those who try to exonerate the executed. And once the person's dead, there's a lot less reason for anyone to push for exoneration.
The second flaw is that it has been shouted — maybe not from rooftops but in the press. Here are ten instances, most of which were established before Scalia's "rooftops" statement in 2006.
The more than 150 people who were sentenced to Death Row but exonerated before execution is also a pretty strong argument that we do execute the innocent. If McCollum had been executed back when Scalia said his lethal injection would be "enviable," his innocence would probably have never been established. But no, Scalia did not say McCollum's guilt was inarguable. He said the guilt of everyone who'd been executed was inarguable.