According to court documents, Rettig and Martin Cameron Bond, both 24, broke into Mortensen's Payson home to steal the man's extensive weapons collection. During the burglary, Rettig trained a handgun on Mortensen as Bond retrieved a knife he used to slash the 70-year-old man's throat, prosecutors said.
Rettig also helped tie up Mortensen's son and daughter-in-law, who happened to come to the house during the burglary.
Rettig's family described him as a kind and peaceful man. In letters to the court, his friends said he once thought of joining the Army but decided against it because he could not take another's life.
But with a gun in his hands, he was "the most powerful person in that house," 4th District Judge Thomas Low said. "Everybody's fate rested in your hands."
"He chose a blood-soaked path," May said.
Mortensen's son, Roger, who along with his wife, spent roughly eight months in jail facing murder charges for the slaying before a tipster led police to Bond and Rettig also spoke prior in court Tuesday.
"It's my understanding that my father was cooperative," Roger Mortensen said. "So why was it necessary to take his life?"
The question has haunted Mortensen's family.
The man's widow said the tragic situation was made worse when prosecutors told her the men were not using drugs and alcohol, and when she found that while they took some guns, her husband's most prized weapons were left untouched.
"I think they had murder on their minds when they went there that night," said Darla Mortensen, the professor's widow.
As part of his plea deal, Rettig has agreed to testify against Bond, whose capital murder trial is scheduled to begin next October. It's that testimony, in part, that led prosecutors to recommend a sentence that included the possibility for parole.
In handing down the sentence, the judge said he believed Kay Mortensen would have wanted to give Rettig the "hope of salvaging" his life should he ever be paroled.
Outside the courtroom, Darla Mortensen, disagreed.
"Knowing Kay, he wouldn't have felt that way," she said and laughed. "He was kind of vindictive and he would have wanted him to pay the price. He [Rettig] took his life."