The court ruled 6 to 2 in the case of Duane Buck. (Drew Angerer/Getty Images)

The Supreme Court on Wednesday said a Texas death row inmate’s sentencing was infected with a “particularly noxious strain of racial prejudice” because of testimony that he might pose a future risk because he is black.

Chief Justice John G. Roberts Jr. cut through a complicated legal background in the case of Duane Buck and said the introduction of expert testimony that said blacks were more violent than others unfairly tainted a jury’s decision about whether he should receive life in prison or death.

In a 6-to-2 decision reopening Buck’s sentencing, Roberts said it did not matter that the introduction of the expert testimony was only a small part of the proceedings.

“When a jury hears expert testimony that expressly makes a defendant’s race directly pertinent on the question of life or death, the impact of that evidence cannot be measured simply by how much airtime it received at trial or how many pages it occupies in the record,” Roberts wrote. “Some toxins can be deadly in small doses.”

This undated photo provided by the Texas Department of Criminal Justice shows inmate Duane Buck. (Texas Department of Criminal Justice via Associated Press)

The two dissenters were Justice Clarence Thomas, the court’s only African American member, and Justice Samuel A. Alito Jr.

They said their colleagues were twisting the law to help Buck.

“Having settled on a desired outcome, the court bulldozes procedural obstacles and misapplies settled law to justify it,” Thomas wrote. “But the majority’s focus on providing relief to petitioner in this particular case has at least one upside: Today’s decision has few ramifications, if any, beyond the highly unusual facts presented here.”

Buck’s lawyer said the outcome should result in a new hearing for the now-53-year-old inmate and send a notice about testimony that touches on race.

The decision “made clear that there is no place for racial bias in the American criminal justice system,” said Christina Swarns, litigation director of the NAACP Legal Defense and Educational Fund, adding that “the court has reaffirmed the long-standing principle that criminal punishments — particularly the death penalty — cannot be based on immutable characteristics such as race.”

Buck went to the home of his ex-girlfriend, Debra Gardner in July 1995 and fatally shot her while her children looked on. He also killed Gardner’s friend Kenneth Butler and shot his own stepsister, who survived.

To say he was unrepentant is an understatement; when arrested, he said Gardner got what she deserved and was “laughing and joking and taunting,” Thomas noted.

Buck was quickly convicted. What was contested at the Supreme Court was what happened at the hearing into whether he deserved the death penalty.

To apply the death penalty in Texas, jurors had to find that the convicted presents a future danger. Buck’s own lawyers called to the stand Walter Quijano, a psychologist who had compiled a report listing race among the factors associated with violent behavior.

Quijano testified that he thought Buck was unlikely to commit future violent attacks but acknowledged that he could “never rule out” the possibility.

Under cross-examination, Quijano suggested that being male and black would make someone more likely to be dangerous in the future. The jury took the report with them before returning to sentence Buck to death.

But Buck’s attempt to argue he deserved a new sentencing hearing because of ineffective counsel hit a wall at the U.S. Court of Appeals for the 5th Circuit, which governs the states of Texas, Louisiana and Mississippi.

A number of procedural mistakes and missed opportunities during what Roberts called a “labyrinth” of state and federal court reviews made it impossible for Buck to raise the ineffective counsel argument, the appeals court ruled.

Roberts said the “extraordinary nature” of the case demanded more.

The decision of Buck’s trial lawyer to call Quijano to the stand “fell outside the bounds of competent representation,” Roberts wrote, but it would have been the same if prosecutors had called him.

“Dr. Quijano’s report said, in effect, that the color of Buck’s skin made him more deserving of execution,” he wrote. “It would be patently unconstitutional for a state to argue that a defendant is liable to be a future danger because of his race.”

He added: “Our law punishes people for what they do, not who they are. Dispensing punishment on the basis of an immutable characteristic flatly contravenes this guiding principle.”

Roberts noted that Texas found six instances of Quijano’s race-based testimony in capital sentencing cases and, nearly two decades ago, voluntarily offered resentencings for all but Buck. (All of them again received the death penalty.)

“In concluding that the value of finality does not demand that we leave the . . . court’s judgment in place, we do no more than acknowledge what Texas itself recognized 17 years ago,” Roberts wrote.

The case is Buck v. Davis.