On Dec. 3, Texas plans to execute an inmate named Scott Panetti, who was convicted in 1995 for murdering his in-laws with a hunting rifle. There is no question that Mr. Panetti committed the murders. There is also no question that he is severely mentally ill, and has been for decades.

During his capital murder trial, at which he was inexplicably allowed to represent himself, Mr. Panetti dressed in a cowboy suit and attempted to subpoena, among others, John F. Kennedy and Jesus Christ. A standby lawyer said his behavior was “scary” and “trance-like,” and called the trial “a judicial farce.”

It was not an act. Mr. Panetti, now 56, was first diagnosed with schizophrenia when he was 20, and in the years before the murders he was hospitalized several times for delusions and psychotic episodes.

In this respect, he is no different from the estimated 350,000 inmates around the country with mental illness — 10 times the number of people in state psychiatric hospitals. But Mr. Panetti is not just another insane prisoner; his name is synonymous with the Supreme Court’s modern jurisprudence about mental illness on death row. In Panetti v. Quarterman, decided in 2007, the justices held that it is not enough for a defendant simply to be aware that he is going to be executed and why — the previous standard the court had used in permitting the execution of the mentally ill. Rather, he must have a “rational understanding” of why the state plans to kill him.