In Slavery in Massachusetts, Thoreau writes:
It is, to some extent, fatal to the courts, when the people are compelled to go behind them. I do not wish to believe that the courts were made for fair weather, and for very civil cases merely; but think of leaving it to any court in the land to decide whether more than three millions of people, in this case, a sixth part of a nation, have a right to be freemen or not? But it has been left to the courts of justice, so called,—to the Supreme Court of the land,—and, as you all know, recognizing no authority but the Constitution, it has decided that the three millions are, and shall continue to be slaves. Such judges as these are merely the inspectors of a pick-lock and murderer’s tools, to tell him whether they are in working order or not, and there they think that their responsibility ends. There was a prior case on the docket, which they, as judges appointed by God, had no right to skip; which having been justly settled, they would have been saved from this humiliation. It was the case of the murderer himself.
What prior legal case does the author have in mind?