Gary Green is executed in Texas for the murder of his wife and her 6-year-old daughter in 2009.

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Texas is due to execute Gary Green on Tuesday after his lawyers unsuccessfully argued that his mental retardation and history of mental illness should disqualify him from the death penalty.

Green, 51, was convicted in 2010 of murdering his wife, Lovette Armstead, and her 6-year-old daughter, Jazmine Montgomery, a year earlier. After Green found out that his wife wanted their marriage annulled, he fatally stabbed Armstead and drowned Montgomery in the tub, according to court documents. Green went to the police and confessed to the murders.

Greene’s lawyers wrote to Dallas County District Attorney John Crusoe in late February asking him to join in their request to delay Greene’s execution so he could undergo more tests for his mental retardation. Creusot did not join the movement.

While experts testified at trial that Green probably had schizoaffective disorder, his lawyers say his defense lawyer did not pay enough attention to how the condition affected his life or what role it played in the murders. Under Texas law, jurors are allowed to consider extenuating circumstances, such as mental illness, when reaching a death sentence. Green appealed to the Texas Court of Criminal Appeals, which upheld his conviction and death sentence.

“Green’s explanation for schizoaffective disorder would help jurors weigh Green’s moral guilt for his crime,” Michael Maula, one of Green’s lawyers, said in a statement. “It is clear from Green’s statements that his mental state at the time of the crime was greatly affected by his severe and persistent mental illness, especially if it was filtered out by his severe cognitive limitations.”

The US Supreme Court in 2002 banned the execution of people with intellectual disabilities. Texas defines mental retardation based on low IQ scores, with 70 generally considered the threshold; how prisoners interact with others and take care of themselves; and whether deficiencies occurred in these areas before the age of 18. The lowest IQ that Green presented in his state trial was 78, placing him in the “borderline” range of intellectual functioning.

Green planned to “claim five lives,” he wrote in a letter to Armstead. According to court documents, he tried to kill Armstead’s sons, then aged 9 and 12, but they convinced him not to. Green then attempted suicide by taking large amounts of Tylenol and Benadryl. When Green turned himself in to police hours later, he said he believed the family were plotting against him.

A month before the murder, Green tried to get help at the Timberlon Mental Hospital in Dallas. He was misdiagnosed, discharged after four days, and was later unable to continue taking his prescribed antipsychotics because of the cost.

Green is also involved in an ongoing legal battle over the state’s use of expired drugs to kill prisoners. With fewer pharmacies ready to produce execution drugs, the Texas Department of Criminal Justice has been extending the expiration dates of lethal injections for years, which may have made the process more painful.

Prisoners say the state’s prison system should not extend the expiration date of its drugs. They argue that the practice violates the US Constitution’s prohibition against cruel and unusual punishment.