By Lawrence Hurley

WASHINGTON (Reuters) - The U.S. Supreme Court on Thursday sided with a death row inmate in a ruling warning of the hazards of judicial bias, slamming Pennsylvania's former chief justice for not stepping aside in a murder case in which he earlier had served as a prosecutor.

The court's 5-3 decision gave Terrance Williams, convicted in the 1984 bludgeoning murder of a 56-year-old man in Philadelphia, a fresh chance to challenge his death sentence before the state's top court due to now-retired judge Ronald Castille's actions.

"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 Anthony Kennedy wrote in the ruling.

Castille, a Republican, served as Philadelphia's district attorney and authorized seeking the death penalty against Williams. When Castille later pursued election to the state Supreme Court, he ran ads touting his record of sending 45 people including Williams to death row. He became Pennsylvania's chief justice and served on the state Supreme Court when it affirmed the death sentence given to Williams.

"Today, Terry Williams comes one step closer to the new, fair sentencing hearing he deserves," said his lawyer, Shawn Nolan.

Lawyers for Williams, a former star high school quarterback who was 18 at the time of the crime, introduced evidence during the appeals process that he had been sexually abused by the man he killed, a church deacon. A lower court then found that prosecutors working for Castille had withheld evidence about the man's sexual abuse of boys.

In 2014, Castille and the other Pennsylvania Supreme Court justices unanimously reinstated the death penalty against Williams after a judge had thrown out the sentence due to the prosecutorial misconduct.

Castille, first elected to the state's high court in 1993, had refused a request by Williams' lawyers to recuse himself.

The U.S. Supreme Court reversed the state's court decision, giving Williams a new hearing before that court to challenge his death sentence.


The ruling made clear to judges that they have to be aware of the potential for the appearance of bias.

"They should think twice about whether the public rather than themselves would consider them neutral or impartial in any given case," said Matt Menendez, a lawyer with the Brennan Center for Justice, a legal think tank in New York.

Kennedy wrote that the U.S. Constitution's "due process guarantee that 'no man can be a judge in his own case' would have little substance if it did not disqualify a former prosecutor from sitting in judgment of a prosecution in which he or she had made a critical decision."

Kennedy was joined by the court's four liberals in the ruling, with conservative justices John Roberts, Clarence Thomas and Samuel Alito dissenting.

"The majority opinion rests on proverb rather than precedent," Roberts wrote in his dissenting opinion.

Speaking to the Philadelphia Inquirer after the ruling, Castille downplayed his role in Williams' prosecution, saying it was "merely procedural." Castille expressed surprise at Kennedy joining the liberal justices in the majority. "He usually understands these criminal justice cases," Castille said.

The decision had echoes of a 2009 ruling Kennedy authored in which the court ruled that a West Virginia appeals court judge should have recused himself in a case in which one of the parties had contributed $3 million to his election campaign.

Pennsylvania currently has a death penalty moratorium imposed by Governor Tom Wolf.

(Reporting by Lawrence Hurley; Editing by Will Dunham)