They said this type of death would being in violation of the constitutional guarantee against cruel and unusual punishment.
These concerns resulted in the killer being granted two previous last minute reprieves from execution in 2014 and 2018, but no such reprieve was granted on Tuesday.
Missouri Department of Corrections took several steps to avoid a botched execution, including sedating him beforehand and elevating him to ensure he would not choke if one of his tumours did burst.
Bucklew was strapped to the gurney and looked around the room and twitched his feet in the moments leading up the execution at 6pm.
As the lethal injection was administered he took a deep breath and stopped all movement. He showed no outwards signs of distress and was pronounced dead at 6.23pm local time.
Bucklew's lawyer, Cheryl Pilate, said while he didn't appear to suffer, there was no real way of knowing.
"The reality is they're completely strapped down, their extremities are completely covered up, which is not a transparent process," she told The Appeal.
"We truly don't know what he experienced."
Morley Swingle, the prosecutor who tried Bucklew in Missouri, said the death "was very peaceful".
Swingle said Bucklew "closed his eyes and went to sleep" and that his death was in stark contrast to the violent acts carried out against his victims.
He said the crimes committed by Bucklew were among the most horrific he had seen in his career.
"He is probably the most pure sociopath I ever prosecuted," Swingle said.
"He was relentless in the way he came after his victims."
BUCKLEW'S CRIMES
The string of violent crimes that led to Bucklew's death penalty conviction started after his girlfriend, Stephanie Ray, broke up with him on Valentines Day in 1996.
Court records show that in the follow weeks he harassed her, with things escalating to him cutting her with a knife and punching her in the face.
The constant harassment and attacks left Ms Ray fearing for her life, so she made the decision to move in with her new boyfriend, Michael Sanders, and his children.
Ms Ray, her new boyfriend and their children all shared a mobile home in the Cape Girardeau County.
But the move only served to enrage Bucklew.
On March 21 he took two pistols, handcuffs and duct tape from his brother, stole his nephew's car and followed Ms Ray back to the mobile home.
All the children were home at the time and Mr Sanders confronted Bucklew with a shotgun inside the home.
Bucklew fired two shots, piercing Mr Sanders' lung and causing him to bleed to death in front of his children.
He ten shot at the man's 6-year-old son but missed.
Court records say he struck his ex-girlfriend in the face with the pistol, handcuffed her and dragged her to his car.
He later raped Ms Ray before fleeing north.
A state trooper spotted Bucklew's car and a gunfight broke out after he pulled him over. Both men were wounded in the shoot out.
Bucklew was locked up in Cape Girardeau County Jail but managed to escape from the facility.
He tracked down Ms Ray's mother and her boyfriend and attacked the couple with a hammer before being recaptured.
EXECUTION 'MOST GRUESOME IN HISTORY'
In the lead up to Bucklew's execution more than 45,000 people signed a petition to stop his "torturous execution".
"What makes Rusty's execution different is that he has a medical condition that would make it one of the most gruesome in US history," the American Civil Liberties Union (ACLU) wrote.
"This is cruel and unusual — and it will result in a bloody, botched execution that will be a stain on not only Missouri, but on this whole country."
There were also concerns that Bucklew had not received an adequate defence before he was convicted.
In 2018 his lawyer's discovered that the medication he was taking for his illness at the time of the crime often caused him to be sent into a rage.
He had since been taken off that medication, with lawyers saying he was remorseful for his crimes.
In April, the Supreme Court ruled against Bucklew in a 5-4 decision, which means plans for the lethal injection could proceed.
Justice Neil Gorsuch said the Eighth Amendment to the Constitution "does not demand the avoidance of all risk of pain" in carrying out executions.
After his execution director of the Capital Punishment Project at the ACLU, Cassandra Stubbs blasted the death as "unnecessary and unlawful".
"Missouri carried out an execution that risked torture and violated international law, despite Mr. Bucklew's remorse and exemplary prison record," she said.
"To think that killing this man — who was terminally ill and had been a model prisoner for 23 years — was worth the stain on our democracy and our humanity is a disgrace."
