Sean Doyle has been credited with capturing one of the world’s most wanted men in Calgary 31 years ago, but it was years before he was free from the traumatic effects of the encounter.

Doyle, a retired art teacher, was working at his summer job as a security guard at The Bay in downtown Calgary one day in July 1985, when he caught a man stealing food.

“I showed him my badge, I said, ‘I’m with Bay security and you’re under arrest,’” Doyle recalled in an interview with CTV Calgary.

It turns out, Doyle had caught Charles Ng, who at the time, was one of the world’s most notorious serial killers.

Ng was accused of murdering and torturing dozens of people, including children, on a ranch in northern California. He fled to Canada and was hiding out in Calgary’s Fish Creek Park.

But that day, Doyle didn’t know he had a serial killer under his arrest. Moments later, it would become apparent how dangerous the situation was.

As he walked Ng back into the store following the arrest, his co-worker screamed, “He’s got a gun.”

Doyle said when Ng pulled the gun out, he grabbed onto the firearm and “pushed the barrel of the gun” away, and bang, it went off.”

Shot through the hand, Doyle still held Ng down until police arrived.

“Two little old ladies in the front, they were from the States, they were clapping and smiling,” Doyle recalled. “They thought this was a show we put on for the (Calgary) Stampede.”

Doyle was hailed a hero for nabbing Ng, but as he learned the details about the man he’d caught, the nightmares began.

“Every single night,” Doyle recalled, he’d dream he was putting Ng “in the crosshairs and pulling the trigger, and I could literally see the bullet ripping into his chest and slamming him into the ground.”

Doyle opposed the death penalty and spoke out against it at Ng’s trial in California. But he couldn’t reconcile his murderous dream, which tore him apart.

“My health started deteriorating and I just couldn’t take it anymore.”

It went on for years until Doyle, a devout Irish Catholic, turned to his faith for salvation.

“I got out of bed and I prayed for the power to forgive (Ng),” Doyle said. “And like that, it was over.”

The transformation was powerful, Doyle has since travelled to California, where Ng remains on death row, hoping to give his message to the killer face-to-face.

The state denied Doyle access to Ng but allowed a letter to be passed to him in jail.

“I learned a very good lesson from this whole experience,” Doyle said. “That forgiving people is my mantra.”

Doyle is now working with a friend to chronicle his journey of forgiveness in a documentary called “Forgive/Ng.”

It is expected to be complete by the end of the year.

With a report by CTV Calgary’s Kevin Green