Kelsey Patterson missed half of his trial after he was ejected from the court due to his delusional outbursts and has not seen his lawyer in eight years because he does not understand "hell law". He is due to die by lethal injection on Tuesday.

In September 1992 Patterson, 50, walked 100 metres from his home in Palestine, eastern Texas, to an oil distribution company and shot the owner in the back of the head.

When the owner's secretary came out to see what had happened, Patterson shot her too. Then he went home, told a friend what he had done, stripped naked apart from a pair of orange socks, and walked the streets waiting for the police to come and get him.

In 1993 a jury found him competent to stand trial and found him guilty of murder, which carries the death penalty.

Patterson, an African-American, had been before the Texas courts before for shooting two co-workers without provocation and hitting another one over the head.

But in each case the state would not prosecute him because he was deemed too mentally ill. Instead he was hospitalised, heavily sedated and later released. On his release he would cease to take his medication.

His family had attempted to hospitalise him in the week he killed but were told he could not be admitted unless there was proof that he was an immediate danger.

"He was convicted because they were scared to death of him," says his lawyer, Gary Hart. "And they believed the mental health system was not going to protect them from him and nor was the criminal justice system unless they did what it asked them to do which was sentence him to death."

All that stands between Patterson and the execution chamber now are appeals before the fifth circuit court of appeals and the supreme court.

The supreme court has already ruled against the execution of people with extreme learning difficulties as a form of "cruel and unusual punishment" but there is no such constitutional protection for the mentally ill.

"Kelsey is not stupid," said Mr Hart. "He is over average intelligence and has a brilliant memory. But he is sick. His execution serves neither the retributive nor the deterrent functions the death penalty was intended for."

The last time Mr Hart spoke properly with Patterson was in December 1996. "Midway through the conversation he said he assumed that I was well versed in hell law," recalls Mr Hart. "I said I'm sorry but they didn't teach that at my law school."

Shortly after that Patterson tried to fire Mr Hart and since then he has refused to sit down and talk with him.

Between 1993 and 2003 Texas, home to 13 % of the US population, has accounted for more than 38% of all executions. African-Americans, who comprise 12% of the US population, account for just over 34% of all those executed.

Statistics show that in Texas African-Americans are twice as likely to be executed if they kill a white person than if they kill an African-American.

"Kelsey fits right into those patterns," says Mr Hart. "But I don't think race was the deciding factor in this particular case."

• An Oklahoma court yesterday postponed the execution of a convicted murderer from Mexico, Osbaldo Torres, granting his request for a hearing on the state's failure to inform him of his right to contact the Mexican consulate after his arrest.The international court of justice in The Hague, ruled in March that the rights of 51 Mexicans on death row had been violated because they were not told they could receive help from their government.