Texas is preparing to execute a mentally ill man who dressed as a cowboy at his trial and attempted to subpoena Jesus Christ, John F Kennedy and the Pope.

On Wednesday, a day after Miguel Paredes became the 10th – and at the time it was thought final – Texas prisoner to be put to death this year, the state gave Scott Panetti an execution date of 3 December. The 56-year-old shot his parents-in-law to death in 1992 in front of his estranged wife and their three-year-old daughter.

Long and complex legal battles have taken place since he was convicted in 1995 as a succession of courts have ruled Panetti is competent to be executed even though there is no question that he is mentally ill and suffers from paranoid delusions.

A schizophrenic man who had been hospitalised 14 times before the murders of Joe and Amanda Alvarado in the Texas Hill Country, he insisted he represent himself at trial, where his only defence was insanity. He said that during the crime he was under the control of a hallucination he called “Sarge”. He wore a purple cowboy suit, rambled incoherently, fell asleep and made a threatening gesture at the jury. Court documents show his standby attorney described Panetti as “bizarre”, “scary” and “trance-like” and that the trial was a “judicial farce”.

The jury convicted him of capital murder and sentenced him to death. The Wisconsin native later filed a federal appeal arguing that he had been incompetent both to stand trial and to represent himself.

At a district court evidentiary hearing, experts testified that Panetti believed the state was “in league with the forces of evil” and wanted to kill him “to prevent him from preaching the Gospel”. One another occasion a doctor testified that Panetti was delusional, believed his planned execution was a satanic plot and talked “about the conspiracies between the Bushes and large corporations and demonic forces”.

Yet courts have found Panetti competent to be executed on the basis that legal standards require only that prisoners know that they are going to be executed, and why, and that “awareness … is not necessarily synonymous with ‘rational understanding’”. Experts for the state who interviewed him concluded that he was at least partially faking his symptoms.

In an effort to boost their case, state attorneys presented in court about 11 hours of audio recordings of conversations between Panetti and his parents and sister that officials secretly taped between December 2007 and January 2008. They argued that these showed Panetti was rational and normal.

Last year the federal fifth circuit court of appeals agreed with the district court’s verdict that Panetti is competent to be lethally injected because he “has both a factual and rational understanding of his crime, his impending death, and the causal retributive connection between the two”, which was shown by his “rationally articulated” belief that his punishment is unfair because he is insane.

The US supreme court looked at his case in 2007, blocked his execution and sent the case back to the district court for re-examination of the vague standards used to determine mental competence. Earlier this month the supreme court refused to consider another appeal.

“Scott Panetti is not competent for execution and therefore his execution would serve no retributive purpose. It is unfortunate that an execution date has been set,” Greg Wiercioch, one of Panetti’s attorneys, said in a statement. “His execution would be a miserable spectacle.”