WASHINGTON – The Supreme Court blocked the execution Thursday of a mentally ill Texas man whose lawyers say he is too delusional to understand the legal process.

Scott Panetti, a paranoid schizophrenic, shot and killed his in-laws 15 years ago in front of his estranged wife and their 3-year-old daughter. He now thinks that Satan is using the state’s penal machinery to stop him from preaching the Gospel.

The case posed the issue of how insane a person must be before a death sentence becomes unconstitutional, and marks the fifth time the Supreme Court has reversed a Texas death penalty since October.

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 that 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 by the federal appeals court that handled the case.

“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 bizarre and satanic motives to the authorities.

Texas Solicitor General Ted Cruz had urged the justices to focus on the central fact – essential to a legal finding of mental competence -- that Mr. Panetti understands that he has been found guilty of murder and faces execution for that murder. Thursday he said in a statement that the ruling “will invite abuse from capital murderers, subject the courts to numerous false claims of incompetency, and even further delay justice for the victims' families.”

Lawyers said a federal trial court will end up determining if Mr. Panetti has sufficient understanding to overcome the Eighth Amendment ban on “cruel and unusual” punishment.

“The Supreme Court recognized that executing Scott Panetti would be a mindless, meaningless, and miserable spectacle,” said Gregory Wiercioch, the Texas Defender Service attorney who argued the case. “The Supreme Court today is recognizing that we don’t put to death people like Scott Panetti who believed his execution would be the result of a satanic persecution rather than state-sanctioned retribution.”

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 legal insanity – leaving lower courts to sort through muddled guidelines.

In 1986, the high court ruled that executing the insane violates the Eighth Amendment ban on "cruel and unusual punishment." No majority managed to settle on specific standards, though. A four-justice plurality asserted broadly that civilized societies don’t execute people who “lack the basic capacity to understand why they are being punished." And Justice Lewis Powell wrote a concurring opinion proposing the rule that no one be executed unless they perceive "the connection between his crime and his punishment” – the standard Mr. Cruz had urged on the court, and which Justice Thomas also said would make sense.

Mr. Panetti’s lawyers derided that as a so-called “bare awareness” test which offers too little protection to someone as deranged as their client.

“The Supreme Court has taken a much-needed step toward a more humane America," Larry Cox, executive director of Amnesty International USA, said after Thursday’s ruling.

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.

The American Bar Association’s task force on mental illness and the death penalty called for a similar standard, and task force member James Eisenberg, a psychology professor and director of the Criminal Justice Program at Lake Erie College, said the court would have dealt the mentally ill “a devastating blow” had it allowed Mr. Panetti’s execution.

“With the conservative majority,” he said, “… it could easily have gone the other way. They could have given short shrift to the issue of mental illness but the majority at this time recognizes that mental illness is real.”

Since October, the Supreme Court has blocked five Texas executions.

Legal experts point both to dissatisfaction over the lower courts’ handling of such cases, and the fact that Texas executes far more inmates than any other state -- 397 since 1976, compared to 1,087 for the entire country, according to the Death Penalty Information Center. There have been 18 executions in Texas so far this year.

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: 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; 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.

The line-up was the same in those cases as on Thursday, with the most conservative justices on the losing side: Chief Justice John Roberts and Justices Thomas, Antonin Scalia and Samuel Alito.

At the time of the Smith, Brewer and Abdul-Kabir trials, 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. The Legislature added a third test in 1991, asking juries to explicitly consider mitigating factors.

The Smith case was handled by the Texas Court of Criminal Appeals. The other two were handled by the Fifth Circuit Court of Appeals in New Orleans – the court Justice Kennedy chastised Thursday for taking a flawed and overly narrow view of mental competence in the Panetti case.

Fifty Texas inmates were sent to death row under the flawed jury instructions, and within a week of the April rulings, the Supreme Court gave a last-minute reprieve to a fourth Texan, 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.

Staff writer Diane Jennings in Dallas contributed to this report.