Yesterday, the Supreme Court made what was a very important ruling.

It said that judges have a constitutional requirement to recuse themselves from cases where they have a significant personal stake:

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On Thursday morning, the Supreme Court decided an actual case of judicial bias, voting 5-to-3 that a judge may not rule on a case in which he previously played a significant and personal role as a prosecutor.

The case before the justices was an appeal by a Pennsylvania man named Terrance Williams, who was convicted of a brutal murder committed in 1984, when he was 18, and sentenced to death.

There was never any question that Mr. Williams committed the crime. But at his trial, he denied knowing his victim, a man named Amos Norwood, and the prosecution’s case was that he had killed Mr. Norwood in the course of robbing him. It later came to light that Mr. Norwood had been sexually assaulting Mr. Williams and other underage boys, and that the prosecutors had known this fact but kept it from the jury.

A trial court agreed that this was prosecutorial misconduct and overturned Mr. Williams’s sentence, but the state supreme court unanimously reversed that decision. And that was where things went wrong, according to the United States Supreme Court. The chief justice of the Pennsylvania supreme court, Ronald Castille — who wrote separately to denounce the trial court’s decision and defend the prosecutors — had been the district attorney who personally approved seeking the death penalty in Mr. Williams’s case.

In 2012, Mr. Williams asked Justice Castille to recuse himself, and the justice refused.

In an opinion by Justice Anthony Kennedy, the court ruled that this violated Mr. Williams’s right to due process, and tossed out his sentence. “Where a judge has had an earlier significant, personal involvement as a prosecutor in a critical decision in the defendant’s case, the risk of actual bias in the judicial proceeding rises to an unconstitutional level,” Justice Kennedy wrote.

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As the majority noted, this ignores not only the significance of the decision Mr. Castille made as a prosecutor, but also the fact that he did not simply sign off on the decision to seek death for Mr. Williams and then forget about it. To the contrary, he was deeply invested in his tough-on-crime reputation as a prosecutor. In his campaign for the state supreme court seat, he boasted about the 45 men, Mr. Williams included, he had sent to death row.

This is a big deal, and Judge Castille should not be allowed to judge a dog show.