If Robert Ladd lived in any other state in the country, he would be spared from execution. But Ladd -- a convicted killer deemed mentally disabled by medical professionals -- will face the death penalty Thursday in Texas, where judges ruled he doesn't meet the state’s threshold for intellectual disability, according to the American Civil Liberties Union, which is representing Ladd’s appeal to halt the imminent execution.

A Texas criminal court sentenced Ladd to death for the 1996 murder of a 38-year-old mentally impaired woman, who was strangled, bludgeoned, bound and burned in her East Texas apartment. At the time of the murder, Ladd was on parole after serving 12 years of a 40-year prison sentence for the 1980 fatal stabbing of a Dallas woman, according to the Associated Press.

Ladd was just hours away from his execution in 2003 when a federal court agreed to hear evidence suggesting he was intellectually impaired. ACLU attorneys cited a Texas psychiatrist’s diagnosis in 1970 that Ladd was “fairly obviously retarded” with an IQ of 67. The psychiatrist later reaffirmed his initial determination in an affidavit, stating Ladd’s IQ test and “three separate interviews confirmed my diagnosis of mental retardation,” according to a statement from the ACLU.

But the courts, including the U.S. Supreme Court last year, have since denied Ladd’s death penalty appeal, arguing the 57-year-old man doesn't meet the Texas courts’ standards for determining mental disability, guidelines which haven't been endorsed by the scientific or medical community. Ladd is scheduled to die Thursday at 6 p.m. CT. The ACLU petitioned the Supreme Court Wednesday for a stay of execution and to review the ruling, according to the statement.

Texas has executed more prisoners than any other state in the country. More than 500 inmates have been put to death since 1982, according to the Texas Department of Criminal Justice.  Jordan Steiker, director of the University of Texas Law School’s Capital Punishment Center, told Al Jazeera he found it “extremely unlikely” the Supreme Court would spare Ladd’s life. “I don’t think the court seems all that eager to police what’s going on in the state courts,” Steiker told Al Jazeera.

The Supreme Court has previously ruled that executing intellectually disabled inmates violated the U.S. Constitution’s ban on cruel and unusual punishment but has given state governments the scope to determine the standards, Al Jazeera reported.

Georgia inmate Warren Hill was put to death Tuesday, after the Supreme Court refuted arguments that the convicted murderer was mentally disabled. The Supreme Court ordered Wednesday the temporary stay of three Oklahoma inmates’ executions, after hearing their arguments last week challenging the state’s lethal injection procedure. The high court will decide whether the procedure breaches the Eighth Amendment’s prohibition of cruel and unusual punishment, Reuters reported.