None deny guilt of murderer, but critics call refusal to intervene an abdication by nation's highest court

Despite clear evidence that he suffered from severe psychosis and mental illness related to his diagnosed schizophrenia, the state of Florida on Monday executed convicted mass murderer John Ferguson by lethal injection.

Though none dispute his guilt in a series of brutal murders in the 1970s, Ferguson's lawyers, mental health experts, and anti-death penalty advocates condemned the state sanctioned killing and the US Supreme Court's refusal to intervene.

"John Ferguson is without a doubt mentally ill," said one of his attorneys Ben Lewis in an interview with the Guardian's Ed Pilkington. "He has a 40-year history of paranoid schizophrenia, we have more than 30 doctors diagnosing him as that over four decades. Yet Florida is close to eviscerating the US supreme court law that makes him ineligible for execution."

The execution is a troubling development for those concerned about how the mentally ill are afforded constitutional protections by states that still implement the death penalty.

As the Christian Science Monitor explains:

The central issue in Ferguson’s emergency appeal to the US Supreme Court was whether the Florida Supreme Court applied the correct judicial test when it earlier decided that Ferguson was fit enough to be executed. The relevant standard was set in a 2007 US Supreme Court decision. The high court declared that it wasn’t enough that a condemned inmate was “aware” that he was about to be executed. Instead, the justices said an inmate must possess a “rational understanding” of the reasons for his pending execution. Defense attorney Handman said that the Florida high court used the wrong test in the Ferguson case and that the federal appeals court failed to hold the state court accountable for the error. “That is not deference; it is an abdication,” he said.

As Pilkington points out, the killing of Ferguson by Florida, which was deemed justified by state courts, "has echoes with the recent case of Warren Hill, who came close to execution in Georgia last month despite the constitutional prohibition of the death penalty for people with intellectual disabilities."

Ferguson has claimed throughout his imprisonment that he is the "immortal prince of God" and remained convinced his incarceration was not because of the brutality of the murders he committed, but because of a conspiracy to limit his powers.

According to a brief to the court filed by mental health groups on Ferguson's behalf,