What prior "case of the murderer himself" does Thoreau have in mind in "Slavery in Massachusetts"?

Question

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?

Answer 1

The "prior case" is not a literal legal case, but a metaphorical one.

First, Thoreau points out that the Supreme Court of the United States has upheld the legality of slavery.

... 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.

The phrase "recognizing no authority but the Constitution" is key. It implies that the Court should have recognized a different, higher authority when making their decision: God's authority.

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 ... It was the case of the murderer himself.

In this passage, the "murderer" is a metaphor for all slave-owners, or the institution of slavery as a whole.

Murder, everyone agrees, is horrible. If a court let a murderer go because they had killed their victim with an "approved" weapon, we would all be horrified and scandalized, because the court would be missing the point entirely. Thoreau argues that the judges of the Supreme Court (and other courts) are making a similar mistake with slavery. They make decisions about whether a very bad thing is being done in the "correct" way, and ignore the bad thing itself.

Supreme Court justices are appointed by the President, but in Thoreau's view, they are also "appointed by God"; meaning, that they only hold their positions because God allows it, and they have a responsibility to uphold true God-given justice, not just human laws. Thoreau is proclaiming that slavery is a grave crime under God's law, and yet the courts are ignoring this "prior case" that God has assigned to them.