Attorneys for a Texas death row inmate with schizophrenia are attempting to stop his execution with an emergency motion, calling for a mental evaluation.

The motion could be one of the last available options to stop the execution on 3 December of Scott Louis Panetti, who has been mentally ill for almost three decades, and believes he is being executed because of a satanic plot carried out by the state of Texas.

“Mr Panetti’s severe mental illness has infected every stage of his capital case,” said Kathryn Kase, one of two attorneys who have represented Panetti for almost a decade. “His execution now would cross a moral line and serve no penological purpose.”

Panetti was sentenced to death for shooting dead the parents of his estranged wife, in front of her and the couple’s three-year-old daughter in 1992. Attorneys argue that Panetti should not be executed because he does not understand why he is being killed. Attorneys say Panetti believes his execution is being orchestrated by Satan through the state of Texas, to stop him preaching the Gospel of Jesus Christ to the condemned.

For almost 20 years, Panetti’s case has been heard from western Texas district courts all the way to the US supreme court, which ruled in 2007 that the case should be re-evaluated. The case has attracted the attention of many high-profile mental health experts, including the American Psychological Association, which wrote a “friend of the court” brief on Panetti’s behalf. The National Alliance on Mental Illness issued an editorial protesting against his execution.

On 16 October his execution was scheduled, after the highest court rejected attorneys’ attempts to clarify standards of competency. Panetti’s attorneys say they only learned his execution had been scheduled through a newspaper notice, two weeks later.

“The state’s conduct would be disturbing if this were a routine case,” said attorney Gregory Wiercioch, who has represented Panetti for about a decade, “but it is unconscionable in a death penalty case where the district attorney himself did not believe Scott Panetti was competent to represent himself at trial.”

Panetti has suffered from mental illness for more than three decades, having begun to show signs of paranoia as early as 1978. In 1986, the Social Security Administration found Panetti too ill to work, and entitled him to government benefits. A few months earlier, his wife told Waco hospital workers that Panetti believed the devil was in their furniture and had therefore buried it in the backyard before nailing the curtains shut. Before the murder of his wife’s parents, he was hospitalised a dozen times.

At his trial, Panetti insisted on representing himself while wearing a TV cowboy costume and a purple bandana. He attempted to subpoena more than 200 people, including the pope, Jesus Christ and John F Kennedy. He fell asleep during trial, rambled incoherently and threatened the jury with an imaginary rifle. His standby attorney called the trial a “judicial farce”.

Despite this, and the supreme court-ordered re-evaluation of the case, in 2008 Panetti was found fit to be killed by the state.

“Panetti was mentally ill when he committed his crime and continues to be mentally ill today,” wrote judge Sam Sparks, for the federal court of the western district of Texas in that 2008 ruling. “However, he has both a factual and rational understanding of his crime, his impending death, and the causal retributive connection between the two.

“Therefore, if any mentally ill person is competent to be executed for his crimes, this record establishes it is Scott Panetti.”

In 2014, the supreme court declined to clarify the insanity standard. In its 2007 ruling, dissenting justices called the ruling “a half-baked holding that leaves the details of the insanity standard for the district court to work out”.

Attorneys’ latest motions ask for Panetti to be re-evaluated before the state executes him. He was last evaluated in 2008.