SCOTUS overturns Pennsylvania death sentence because involved DA who became state justice did not recuse

A death row defendant in the Keystone State got a key win on a judicial bias claim from SCOTUS this morning in Williams v. Pennsylvania, No. 15-5040 (S. Ct. June 9, 2016) (available here). Justice Kennedy authored the opinion for the Court, while Chief Justice Roberts dissented in an opinion Justice Alito joined and Justice Thomas authored his own dissenting opinion. Here is how the Court's opinion gets started: 

In this case, the Supreme Court of Pennsylvania vacated the decision of a postconviction court, which had granted relief to a prisoner convicted of first-degree murder and sentenced to death.  One of the justices on the State Supreme Court had been the district attorney who gave his official approval to seek the death penalty in the prisoner’s case.  The justice in question denied the prisoner’s motion for recusal and participated in the decision to deny relief.  The question presented is whether the justice’s denial of the recusal motion and his subsequent judicial participation violated the Due Process Clause of the Fourteenth Amendment.

This Court’s precedents set forth an objective standard that requires recusal when the likelihood of bias on the part of the judge “‘is too high to be constitutionally tolerable.’” Caperton v. A. T. Massey Coal Co., 556 U.S. 868, 872 (2009) (quoting Withrow v. Larkin, 421 U.S. 35, 47 (1975)).  Applying this standard, the Court concludes that due process compelled the justice’s recusal.