Johnny Johnson adbucted and killed six-year-old Casey Williamson (Picture: AP/Reuters)

A ‘vampire’ killer told his six-year-old victim’s family he was ‘sorry’ and sipped on a milkshake before being executed.

Johnny Johnson, 45, received a lethal injection dose of pentobarbital at a prison in Bonne Terre, Missouri, Tuesday evening.

He was pronounced dead at 6:33pm CDT, the authorities said according to the Associated Press (AP).

In his final handwritten statement, Johnson said: ‘God Bless. Sorry to the people and family I hurt.’

His last meal consisted of a bacon cheeseburger, curly fries and a strawberry shake.

Johnson, who had schizophrenia, was executed for the killing of Casey Williamson, a decision that ignited a years-long legal battle as lawyers and human rights groups said he was not legally competent enough for the death penalty.

Johnson’s execution saw Supreme Court justicies and the Pope push and tug over whether the move was the right one (Picture: AP)

Turning his head to speak to his spiritual adviser, Johnson was motionless as he was injected and looked blanket forward.

Members of Casey’s family were among those witnessing him becoming the fourth inmate to die from lethal injection in the state this year.

According to court documents, Johnson swung by a barbecue the night before Casey Willamson’s murder on July 25 2002 at her father’s home in the quiet Valley Park suburb and spent the night on his sofa.

Angie Wideman, Casey’s mum, told the St. Louis Post-Dispatch that Johnson was her best friend’s younger brother.

Johnson the following morning lured Casey – still in her nightgown – into an abandoned glass factory with the intention of assaulting her. He even gave her a piggyback ride on the way to the downtrodden site.

Casey, who would have turned 28 this November, fought back only to be beaten several times in the head with a brick and a boulder, killing her.

He lured the girl to an abandoned factory with the intention of sexually assaulting her, court papers said (Picture: AP)

Confessing to the killing, Johnson said he buried the young girl under rocks and leaves less than a mile from her home and washed himself off in the Mermeac River.

He was found guilty of first-degree murder, as well as armed criminal action, kidnapping and attempted rape.

The state governor, Mike Parson, confirmed Johnson’s execution after denying clemency on Monday. He said his office received ‘countless letters seeking justice for Casey’.

‘Johnny Johnson’s crime is one of the most horrific murders that has come across my desk,’ he said.

‘Casey was an innocent young girl who bravely fought Johnson until he took her life.’

‘Although this won’t bring her back, we hope that carrying out Johnson’s sentence according to the Court’s order may provide some closure for Casey’s loved ones,’ he added.

Johnson had long lived and struggle with severe mental health conditions (Picture: KTVI)

Johnson’s execution was briefly stalled as his legal team argued their client had little grasp of the punishment fitting the crime.

His lawyers said he was ‘in the grips of psychosis’ when he killed Casey, with appeals detailing how he believed he was a ‘vampire’ while behind bars.

‘Johnson has delusions about the devil using his death to bring about the end of the world,’ his attorneys said, according to the non-profit Death Penalty Information Center.

How state officials carried out a competent test was flawed, Johnson’s lawyers argued, with their team’s neuropsychiatric finding Johnson to be incompetent.

A US Supreme Court ruling in 2012 said the death penalty should not be used if the inmate suffers from severe mental health conditions.

In April, Pope Francis’ top representative, Archbishop Christoph Pierre, wrote to the governor to grant clemency for Johnson ‘on the basis of his, and our own, shared humanity’.

Casey’s family said that the griev of losing her has devasted them for years (Picture: Getty Images)

‘Our faith teaches us that every human life is made in the image and likeness of God,’ Pierre said.

But the Missouri Supreme Court denied Johnson’s appeal in June, with the Missouri Attorney General’s Office adding that Johnson is competent enough to live with schizophrenia if he takes his medication.

Johnson’s attorneys had argued otherwise. He had been diagnosed with serious mental disabilities since childhood also including schizoaffective disorder and major depressive disorder.

Before Casey’s murder, Johnson had stopped taking the drugs because they made him ‘feel like a zombie’ after he was released from a mental health facility in January 2002.

As the topsy-turvey legal battle continued, a three-judge federal appeals court panel temporarily halted the execution on July 25 only for the 8th US Circuit Court of Appeal to reinstate it Sunday evening.

On Monday, the Catholic Mobilizing Network (CMN) which campaigns against the death penalty, denounced the court’s decision.

The little girl tried to break free during the sexual assault (Picture: KTVI)

‘All executions violate human dignity,’ the group said, ‘but in cases like Johnny’s, we see how the death penalty also targets some of the most vulnerable among us.’

In a final bid, Johnson’s lawyers had asked the US Supreme Court across three petitions to ‘prevent the execution of a man who is profoundly insane’.

But they were dealt a final legal blow when the six-justice conservative majority rejected the stay of execution.

‘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 and two other justices dissenting wrote yesterday.

‘There is no moral victory in executing someone who believes Satan is killing him to bring about the end of the world.’

Casey’s great aunt, Della Steele, pleaded to the state governor to carry out the execution to ‘send a message that it is not okay to terrorise and murder a child’.

Grief, she said, had ripped through the youngster’s family for more than two decades.

‘He took a life away from a completely innocent child,’ she told AP, ‘and there have to be consequences for that.’

Missouri has executed three other people this year: Amber McLaughlin, Leonard Taylor, and Michael Tisius.

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‘He took a life away from a completely innocent child.’