A Texas death row inmate who tried to call Jesus Christ and John F. Kennedy as trial witnesses is not competent to be executed, a federal judge has ruled.

Scott Panetti, 65, who has been on death row for nearly 30 years for shooting his in-laws dead in front of his wife and young children, claims Texas wants to execute him to cover up incest, corruption, sexual abuse and drug trafficking he has uncovered.

He also said the devil has “blinded” Texas and is using the state to kill him to stop him from preaching and “saving souls”.

In a ruling issued on Wednesday, US district judge Robert Pitman in Austin said Panetti’s well-documented mental illness and disorganised thought prevent him from understanding the reason for his execution.

The US Supreme Court has banned the death penalty for the intellectually disabled but not for people with serious mental illness.

However, it has ruled that a person must be competent to be executed.

“There are several reasons for prohibiting the execution of the insane, including the questionable retributive value of executing an individual so wracked by mental illness that he cannot comprehend the ‘meaning and purpose of the punishment’, as well as society’s intuition that such an execution ‘simply offends humanity’. Scott Panetti is one of these individuals,” Mr Pitman wrote in his 24-page ruling.

Panetti’s lawyers have long argued that his 40-year documented history of severe mental illness, including paranoid and grandiose delusions and audio hallucinations, prevents him from being executed.

Gregory Wiercioch, one of Panetti’s lawyers, said Mr Pitman’s ruling “prevents the state of Texas from exacting vengeance on a person who suffers from a pervasive, severe form of schizophrenia that causes him to inaccurately perceive the world around him”.

“His symptoms of psychosis interfere with his ability to rationally understand the connection between his crime and his execution. For that reason, executing him would not serve the retributive goal of capital punishment and would simply be a miserable spectacle,” Mr Wiercioch said in a statement.

The Texas Attorney General’s Office, which argued during a three-day hearing in October that Panetti is competent for execution, did not immediately reply to an email seeking comment on Mr Pitman’s ruling.

Panetti has had two prior execution dates — in 2004 and 2014.

In 1986, the Supreme Court ruled the eighth amendment bars the execution of mentally ill individuals who do not have a factual understanding of their punishment.

In 2007, in a ruling on an appeal in Panetti’s case, the high court added that a mentally ill person must also have a rational understanding of why they are being executed.

At the October hearing, Timothy Proctor, a forensic psychologist and an expert for the state, said that while he thinks Panetti is “genuinely mentally ill”, he believes he has both a factual and rational understanding of why he is to be executed.

Panetti was condemned for the September 1992 slayings of his estranged wife’s parents, Joe Alvarado, 55, and Amanda Alvarado, 56, at their Fredericksburg home in the Texas Hill Country.

Despite being diagnosed with schizophrenia in 1978 and admitted to hospital more than a dozen times for treatment in the decades before the deadly shooting, Panetti was allowed by a judge to serve as his own lawyer at his 1995 trial.

At his trial, Panetti wore a purple cowboy outfit, flipped a coin to select a juror and insisted only an insane person could prove insanity.