* Execution set for 6 p.m. EDT (2200 GMT
By David Adams
MIAMI, Aug 5 (Reuters) - A Florida man who has spent 35 years on Florida's death row for killing eight people, including two teenagers, is scheduled to be executed by lethal injection on Monday pending a last-minute appeal by lawyers claiming that he is insane.
John Errol Ferguson, 65, who was convicted of first-degree murder and sentenced to death in 1978 for a pair of killing sprees, is due to be executed shortly after 6 p.m. EDT (2200 GMT) at Florida State Prison near Starke, Florida.
The National Alliance on Mental Illness (NAMI) filed an amicus brief last week, along with three Florida mental health organizations, asking the U.S. Supreme Court to halt the execution, arguing that Ferguson had a long history of severe mental illness.
The brief argues that Ferguson's execution would violate the Eighth Amendment to the U.S. Constitution requiring an individual to have a rational understanding of why he is being put to death and the effect of the death penalty.
"The death penalty is not constitutionally allowable as a punishment for John Ferguson because his delusions prevent him from understanding the nature of what is happening to him," said Ron Honberg, NAMI's national director of policy and legal affairs.
"The constitutional principle does not excuse his crimes, but it does point to life without parole as the appropriate sentence," he added in a statement.
In July 1977, Ferguson fatally shot six people execution-style during a drug-related home robbery in a northern Miami suburb. Six months later, he killed two teenagers after they left a church meeting.
State psychiatrists and other medical professionals have diagnosed Ferguson as a paranoid schizophrenic with a long history of mental illness, according to his defense team.
Defense attorney Christopher Handman has said Ferguson considers himself "the Prince of God" and does not understand the death penalty and why it had been imposed.
"It is impossible to fathom that the state can constitutionally put to death a man who thinks he is the 'Prince of God' and who believes he has a destiny of being the right hand of God and returning to purify earth after the State tries to kill him," Handman said in a statement before Ferguson's previously scheduled execution last October.
A last-minute court ruling put that on hold amid arguments that he was too mentally ill to be executed.
Courts, however, have repeatedly rejected the claims.
Florida Governor Rick Scott signed Ferguson's death warrant in September, but a few weeks later delayed the execution while a team of physicians met to decide whether Ferguson was mentally competent.
After a 90-minute examination and brief consultation a panel of psychiatrists determined that Ferguson was sane. A state circuit judge agreed in a ruling.
The U.S. 11th Circuit Court of Appeals in May rejected his appeal, ruling that Ferguson was mentally competent.
"That most people would characterize Ferguson's Prince-of-God belief, in the vernacular, as 'crazy' does not mean that someone who holds that belief is not competent to be executed," the appeals court found.
Posing as a utility company worker, Ferguson entered the home of Margaret Wooden, 24, on July 27, 1977 in the Miami suburb of Carol City. He pulled a gun, tied her up and called two accomplices into the home to search for drugs and money.
Six of Wooden's friends arrived at the house soon after and were tied up. Her boyfriend, Michael Miller, 24, also turned up. Six of the victims, including Miller, were shot to death. Wooden and another person escaped.
Ferguson was also found guilty of the abduction and killing of two 17-year-old Miami high school students.
According to court records, Ferguson abducted Belinda Worley and her boyfriend, Brian Glenfeldt, after they left a social event at a church on January 8, 1978.
Their bodies were found the next day in a nearby wooded area. Glenfeldt was shot in the head, chest and arm. Worley was found in dense brush with a gunshot to the back of the head. An autopsy revealed she had been raped. (Editing by Leslie Gevirtz)