Originally Published: 02 AUG 23 13:53 ET

By Dakin Andone, CNN

(CNN) — Missouri has executed a death row inmate for the 2002 murder of a young girl over the objections of his attorneys, who argued he was mentally incompetent and thus ineligible to be put to death.

Johnny Johnson, 45, was sentenced to die for murdering 6-year-old Casey Williamson after abducting and trying to rape her, court records show. Republican Gov. Mike Parson denied a clemency request Monday, calling the crime “one of the most horrific” ever to cross his desk.

Johnson was put to death Tuesday evening by lethal injection, reported the St. Louis Post-Dispatch, a CNN affiliate. He was pronounced dead at 6:33 p.m. CT, a spokesperson for the Missouri Department of Corrections confirmed to CNN.

He was the fourth inmate executed this year in Missouri and the 16th across the United States, according to the Death Penalty Information Center.

Johnson apologized to his victim’s family in a final statement released by corrections officials: “God Bless(.) Sorry to the people and family I hurt,” he wrote.

Casey’s family did not make a statement after Johnson’s execution, according to the Post-Dispatch. Family members had mixed feelings about the execution, the paper reported, but Casey’s mother was ready to put it behind her.

“I’ve been looking forward to putting this part of it to rest,” Angie Wideman said last week. CNN has reached out to her.

Johnson’s lethal injection was carried out after the US Supreme Court denied his request for a stay of execution, with the three liberal justices dissenting.

“The Court today paves the way to execute a man with documented mental illness before any court meaningfully investigates his competency to be executed,” Justice Sonia Sotomayor wrote in her dissent, citing in part Ford v. Wainwright, in which the court ruled the Eighth Amendment prohibits the execution of a person who can’t understand what’s happening. Also dissenting were Justices Elena Kagan and Ketanji Brown Jackson.

A neuropsychiatrist who evaluated Johnson in February determined he long had “suffered from severe mental illness,” including schizophrenia, according to the inmate’s petition for a writ of certiorari.

Johnson understood he was sentenced to die for murdering a child, the doctor found, but he lacked a “rational understanding of the reasons for his execution” and believed that “Satan is using the State of Missouri to execute (him) to bring about the end of the world,” the petition said. Ultimately, the doctor found Johnson incompetent to be executed.

The state opposed Johnson’s petition, citing the findings of his prison’s head of mental health, who reported Johnson had never “expressed these kinds of hallucinations or delusional beliefs.”

Sotomayor wrote the Supreme Court of Missouri and the Eighth Circuit Court of Appeals both had erred in denying Johnson the opportunity to argue his case.

The state Supreme Court first denied Johnson’s request for a competency hearing because it found he had not demonstrated the threshold showing of insanity required; a federal district court then denied relief.

Then, a three-judge panel of the Eight Circuit Court issued Johnson a stay and certificate of appealability, giving him the chance to have his case heard. But the full Eighth Circuit subsequently vacated the stay and declined to issue the certificate.

Johnson “deserves a hearing where a court can finally determine whether his execution violates the Eight Amendment,” Sotomayor wrote in her dissent. “Instead, this Court rushes to finality, bypassing fundamental procedural and substantive protections.”