"He was sick of this life," Kawai said outside of court Tuesday. "He wanted out, and he wanted to take his most precious person with him."
But Kawai said the murder-suicide plan unraveled that April day because after Petersen shot the infant in the basement of his home near 500 East and 500 North in American Fork, Ryker didn't die immediately.
So instead of turning the gun on himself, Petersen called his grandmother for help.
"I would imagine in that situation, his fatherly instinct kicked in," Kawai said. "I would try to save the child."
Police have said Petersen shot Ryker with a .22-caliber rifle then tried to turn the gun on himself, but a family member reportedly intervened.
After the shooting, Petersen was arrested and charged in 4th District Court with aggravated murder, a first-degree felony a death-penalty eligible offense.
Petersen pleaded guilty as charged Tuesday afternoon, his voice cracking as he answered 4th District Judge Darold McDade's questions about waiving his trial rights.
As part of a plea deal, both defense attorneys and prosecutors are asking McDade to sentence Petersen to life in prison without the possibility of parole on Oct. 21.
Before Tuesday's hearing began, Petersen sat in the jury box and cried as family members and Ryker's mother wept in the courtroom gallery. It was the first time Petersen had seen his child's mother since he was arrested, Kawai said.
"He's devastated by what he did," Kawai said.
Kawai said that during his first meeting with Petersen, his client told him he wished for the death penalty. But, after it was explained to him that he would have to go forward with a full-blown trial filled with testimony and crime scene photographs he began considering other options.
"He just wanted to spare his family, Amanda [Ryker's mother] and his grandma the experience of going through that," Kawai said.
Deputy Utah County Attorney Craig Johnson said Tuesday's resolution brought some closure to Ryker's family.
"Nothing we do here will bring Ryker back," Johnson said outside of court. "But the possibility of parole hearings or appeals [with a death penalty case]… That's nothing the family wanted to endure."
The guilty plea came one day after Petersen was ruled competent to stand trial on the charge. Kawai had asked for the evaluation after several encounters with his client left him questioning whether Petersen was suffering from a mental disorder that was preventing him from understanding the charges against him.
"I needed to make sure that Josh was in the right state of mind to make a decision regarding the rest of his natural life," Kawai said Tuesday, noting the plea offer has been on the table since April.