UPDATE January 29, 2015 8:39 EST: Ladd was executed Thursday night by lethal injection in Huntsville, Texas. In his final statement, according to the Associated Press, Ladd addressed the sister of his victim by name and said, "I really, really hope and pray you don't have hatred in your heart. A revenge death won't get you anything." Ladd then told the prison warden, "Let's ride." He died at 7:02 pm CT, 27 minutes after the pentobarbital was administered.

Texas is set to execute convicted murderer Robert Ladd Thursday evening, a man that by most standards would be considered intellectually disabled and ineligible for the death penalty — but not in Texas, where guidelines for culpability are based in part on the classic novella Of Mice and Men.

Barring a last minute intervention by the Supreme Court, Ladd will be put to death by lethal injection at 6pm CT today, capping a decades-long legal battle centering on longstanding claims, including analysis from state health officials dating from the time Ladd was 13, that he is at least mildly retarded.

The 57-year-old was sentenced to death for the 1996 murder of Vickie Ann Gardner, who was bound, beaten with a hammer, and then set on fire during the course of a robbery. Ladd had earlier been convicted of murdering his cousin and her two young children in Dallas, but was released on parole shortly before Gardner's death.

In 2003, Ladd was scheduled to be put to death when a federal court agreed to address a mental capacity appeal. In December, Ladd's date of execution was again postponed on a technicality. Late on Tuesday, The Texas Court of Criminal Appeals — shortly after it issued a stay in another death penalty case — ruled, without explanation, that an intellectual disability appeal filed on Ladd's behalf didn't pass muster, and his execution will go ahead.

In 1970, at 13 years old, Ladd was diagnosed by a Texas State psychiatrist as having an IQ of 67. Ladd, said the doctor, was "fairly obviously retarded."

Later that year, the psychiatrist said he did "not feel that this boy needs any more follow-up sessions due to his limited IQ and motivation to improve himself."

"He was placed in a mental retardation center as an adult, and he had someone who drove him to work," Cassandra Stubbs, director of the ACLU's Capital Punishment Project, which has filed an appeal before the Supreme Court, told VICE News. "He couldn't buy clothes because he didn't know his size. When he tried to pay his bills, he used the wrong amount of money, and someone had to help pay his bills. This is a person with lifelong intellectual disability."

In 2002, the Supreme Court ruled that executing someone with mental retardation was unconstitutional, but left the definition of such incapacity for states to decide. Texas made its determination in part based upon John Steinbeck's Of Mice and Men, which depicts a mentally challenged man, Lennie, who accidentally kills a woman. To avoid being lynched by a vigilante mob, his friend, George, kills Lennie himself.

Steinbeck's story — which for many shows why a mentally incapacitated person shouldn't be held fully accountable for committing crimes — was cited by Texas Criminal Court of Appeals Judge Cathy Cochran as part of a 2004 ruling in a separate death penalty case.

"Most Texas citizens might agree that Steinbeck's Lennie should, by virtue of his lack of reasoning ability and adaptive skills, be exempt from execution," wrote Cochran. "But does a consensus of Texas citizens agree that all persons who might legitimately qualify for assistance under the social services definition of mental retardation be exempt from an otherwise constitutional penalty?"

Texas's reasoning, that not all people considered mentally incapacitated — even by state officials — should be afforded the protections handed down by the Supreme Court, stunned death penalty advocates.

The Supreme Court, however, has shown an unwillingness to intervene in the path states chose to pursue in light of their 2002 decision. Only this week, the court declined to stop the execution of Georgia inmate Warren Hill, whose attorneys had filed a similar appeal with the justices based on claims of mental incapacity.

In 2012, Texas executed Marvin Wilson, a convicted murderer who had an IQ of 61, even lower than Ladd's. Wilson, like Ladd, was a black man. In the 2012 case, the Supreme Court rejected a final appeal from Wilson's lawyers.

Hours before Texas's highest court rejected Ladd's last-ditch effort this week, they did postpone the death sentence of another man, Garcia White, who had been scheduled to die on Wednesday.

Last week, White's attorney Patrick McCann filed an appeal with the high court on the grounds that his client was mentally incapacitated — possibly resulting from crack addiction — when he waived his right to attorney and confessed to police. The defense also argued that DNA evidence suggested another suspect might have had a hand in the murders.

The Texas Court of Criminal Appeals affirmed White's conviction in 1998 for the stabbing deaths 16-year-old twins Annette and Bernette Edwards. The girls allegedly happened upon White fighting with their mother, Bonita, who was also murdered.

In addition to those 1989 killings, White confessed to police he had on separate occasions beaten to death a convenience store clerk and a woman in a drug den.

White's defense team claimed the former high school football player was low functioning and presented studies showing dependence on drugs like cocaine could lead to psychosis. Following his conviction, witnesses testified that White became violent when he used drugs.

McCann told VICE News he spoke with White at 10am on Tuesday, prior to the court's decision. On two previous occasions it had rejected appeals filed on behalf of White, and all signs pointed to the execution being carried out.

In the early afternoon, however, when White's family was saying their final goodbyes, McCann phoned them with the court's ruling.

"They actually got him the news, they were there to say goodbye," said McCann. "It's not often you get to give news like that. There were a lot of joyful tears and screaming when I talked to them, and I'm pretty sure there was the same when they told him."

The Texas court gave no explanation for its decision. The court could still ask for further briefings on the case, or decide one of the issues raised by McCann meets the requirements for sending it back to trial court for additional evidence gathering.

"It's fairly rare to get this out of the Court of Criminal Appeals," said McCann. "My hope is the judges want to hear a good deal more."

Both Ladd and White are plaintiffs in a separate federal suit that claims the supplies of the drug used by Texas to carry out executions, pentobarbital, "are no longer viable and will cause excruciating pain."

The Texas attorney general's office has said those concerns are unfounded, and the state has continued to use the drug as recently as last week.

Though it may emerge that White's reprieve was based entirely on the DNA evidence and not his alleged mental incapacity, advocates say opaque 11th hour consideration of life and death, particularly in cases of possible retardation, is alarming.

"Instead of a rush to look at these questions in the heat of moving forward with an execution, there needs to be a period of time where we stop and look collectively at these issues," Diann Rust-Tierny, executive director of the National Coalition to Abolish the Death Penalty, told VICE News.

"The Supreme Court has been clear, but states like Texas and Georgia keep trying to push the envelope," she said. "What is served by executing a person the constitution has determined doesn't possess the culpability or mental capacity to line up with any of the justifications for why you would have a punishment like this?"