He pleaded guilty to attempted threat of terrorism, a charge that could carry a possible sentence of 15 years in prison.
Stiles didn't have any weapons but court records stated he was planning to buy guns and ammunition. Gill said he took the threat seriously in the aftermath of a mass shooting at the Washington Navy Yard.
Stiles quietly accepted the plea deal on May 5, less than a month after his defense attorneys accused the district attorney's office of keeping him in jail under pressure from police and the shopping center after the case attracted attention.
"Much was made over nothing in this case," said defense attorney Neal Hamilton, who maintained that Stiles could not have carried out the plot and is responding well to therapy.
Gill declined to comment on the defense claim but said he attends the weekly hearings when Stiles checks in with court supervisors.
"We ended up at the right place," Gill said. "The challenge that you have in these type of cases ... is you don't want to guess wrong."
A lawsuit filed by City Creek Center was settled in January after Stiles agreed to permanently stay away from the mall. Attorneys originally sought about $300,000, saying publicity made store owners and shoppers worried about their safety.
City Creek made a contingency plan with police to respond if Stiles ever enters the property, mall general manager Linda Wardell said.
"For us, it's been business as usual throughout the situation," she said.
City Creek is a $1.7 billion shopping center built by the business arm of The Church of Jesus Christ of Latter-day Saints. Opened in 2012, it spans two city blocks across the street from the Mormon temple and features a retractable roof and a creek.
Stiles was in jail for about seven months until the plea agreement.