Eric Robert plead guilty to killing a prison guard during a failed escape attempt in April of last year.
He's waved his right to appeals and even told a judge that if he's kept alive, he will kill again.
Robert was five years into an 80-year sentence for kidnapping.
Monday we talked to the state's attorney who helped prosecute the case in Meade County back in 2005.
"He sat in the courtroom, quiet and respectful of the process," said deputy state's attorney Tracey Decker.
Prosecutors outlined the case against Robert of Piedmont. Decker recalled Robert managed to pull over an 18-year-old girl near Blackhawk.
"He approached her and indicated he was a plain-clothes police officer," she said. "He had her get out of her car and perform field sobriety tests, which is how he got her back to her trunk."
He then forced her into her own trunk and started to drive away.
The girl was able to call police with a cell phone, and officers found the car with her still inside parked on the side of the road about a mile from where she was abducted.
"We were convinced that he meant to sexually assault this girl that he had put in her trunk," said Decker.
Attorneys pointed to duct tape and rope in Robert's car as evidence. He was sentenced in January 2006 to 80 years in prison.
Five years later, he plead guilty to killing prison guard Ronald Johnson.
"I don't think it ever crossed anyone's mind at the time that he would do something like this."
Robert and alleged accomplice Rodney Berget killed the guard during a failed escape attempt.
"It still surprised me a little bit that he was involved in that," Decker said.
The men beat Johnson with a lead pipe and wrapped his head in plastic wrap.
"I've been doing this job long enough that I think, you know, people are capable of just about anything."
The escape failed when Robert, dressed in Johnson's uniform, was stopped at the gate.
Robert plead guilty in September of 2011, and was sentenced to die by lethal injection Monday at 10 p.m.
"I hope that the victim's family can maybe find some peace with it eventually," Decker said, "although they're still missing their loved one."
And despite Robert's admission of guilt, several groups planned to hold a vigil for him to protest the penalty at the state prison starting two hours before his execution.
Berget also plead guilty in the killing, but is seeking an appeal of his death sentence.
Robert and Berget are two of five men currently sitting on South Dakota's death row.
Also on death row is Donald Moeller, sentenced in the rape and murder of a 9-year-old girl. He's set to be executed the week of October 28.
Charles Rhines was sentenced to death in 1993 for murder, and Briley Piper waits to be executed for his role in the same murder that resulted in Elijah Page's execution in 2007.