The case posed the issue of how insane a person must be before a death sentence becomes unconstitutional, and the ruling came at the end of the Supreme Court's term.

At trial, Mr. Panetti, now 49, insisted on representing himself. He wore a purple cowboy costume, tried to subpoena Jesus, the pope and John F. Kennedy, and testified in the persona of his alter ego "Sarge." He'd been hospitalized 14 times for mental illness in the decade before the murders, at his in-laws' Fredericksburg home.

Lawyers handling his appeal argued few, if any, death row inmates are as mentally incompetent and that putting him to death would amount to "mindless vengeance" with no retributive purpose.

Four lower courts did find him competent to stand trial, and a jury rejected his plea of not guilty by reason of insanity.

But Justice Anthony Kennedy, writing for a 5-4 majority, found that Mr. Panetti's mental illness is so severe that it should have been considered.

"Someone who is condemned to death for an atrocious murder may be so callous as to be unrepentant; so self-centered and devoid of Compassion as to lack all sense of guilt; so adept in transferring blame to others as to be considered, at least in the colloquial sense, to be out of touch with reality," Justice Kennedy wrote, but Mr. Panetti's problem isn't merely "a misanthropic personality or an amoral character. It is a psychotic disorder."

At oral arguments in April, justices wrestled with the puzzling situation of an inmate who knows he's been convicted, knows what he's convicted for, knows the state plans to punish him – but suffers from a delusion that makes him ascribe satanic motives to the authorities.

Texas Solicitor General Ted Cruz, arguing for the prosecution, urged justices to focus on the central fact – essential to a legal finding of mental competence -- that Mr. Panetti understands that he is guilty of murder and that he faces execution for that murder.

The American Psychological Association, the American Psychiatric Association and the National Alliance on Mental Illness had all urged the Supreme Court to spare Mr. Panetti, arguing that regardless of current legal definitions, if a person has a mental disorder that "significantly impairs his or her capacity to understand the nature and purpose of the punishment," that person isn't competent to be executed.

Gregory Wiercioch, an attorney with Texas Defender Service who argued Mr. Panetti's case, lauded the ruling.

"The Supreme Court recognized that executing Scott Panetti would be a mindless, meaningless, and miserable spectacle," he said. "The Supreme Court today reaffirms the wisdom of a legal principle nearly a thousand years old – that the execution of persons like Scott Panetti serves no purpose and offends our sense of decency and common humanity."

Justice Clarence Thomas, writing for the four dissenters, criticized the majority's refusal to defer to lower courts regarding Mr. Panetti's mental competence, especially in the absence of evidence that his condition has worsened since his conviction – a contention Mr. Panetti's lawyers dispute.

Justice Thomas accused the court's majority of offering a "half-baked holding" that doesn't clarify standards for findings of legal insanity – leaving, instead, muddled guidelines for lower courts to sort through.

He argued that the only workable standard would be the one offered 20 years ago by Justice Lewis Powell, in a concurring in another mental competency case that left the court even more badly divided. Justice Powell wrote that no one should be executed unless they perceive "the connection between his crime and his punishment" – the standard Mr. Cruz had urged on the court.

The Panetti case is one of a number of Texas death row cases the Supreme Court has scrutinized this term.

In April, a 5-4 court threw out sentences for three killers whose juries, in choosing between execution and life in prison, weren't explicitly allowed to weigh mitigating factors such as childhood abuse.

The court's finding that meant a reprieve for three Texans, including LaRoyce Smith, who killed the night manager at a Dallas Taco Bell where he'd worked. He was a 19-year-old ninth-grader with an IQ of 78.

At the time of trial, Texas law gave juries a two-part test: if the conduct was deliberate, and the defendant posed an ongoing threat to society, the sentence would be death. Those rules were in use during the late 1980s and early 1990s. The Legislature added a third test in 1991, asking juries to explicitly consider mitigating factors.

The Supreme Court threw out the Smith sentence on a 7-2 vote in 2004, but the Texas Court of Criminal Appeals defied the justices, asserting as it had previously that the lack of a the third jury instruction was harmless. The Supreme Court rejected that snub in April. Justice Anthony Kennedy, writing for the majority, caustically asserting that the Texas court suffered "confusion" and an inability to follow directions.

At the same time it threw out the Smith death penalty, the Supreme Court gave reprieves to two other Texas inmates whose appeals on the grounds had come up through the federal courts: Brent Ray Brewer, who stabbed a 66-year-old man and robbed him of $140, and Jalil Abdul-Kabir, who robbed a 66-year-old man who was walking his dog in San Angelo in 1988, strangled him with the leash and got $20.

Fifty Texas inmates were sent to death row under the flawed jury instructions, and within a week of the Smith ruling, the Supreme Court gave a near last-minute reprieve to another of them, Ronald Chambers, a Dallas man who has spent 31 years awaiting execution for a 1975 abduction-murder of a college student. He'd been convicted and sentenced to death three times.

The cases led many legal experts to view a growing sense of dissatisfaction on the Supreme Court over the way Texas has handled the death row process.