In February 1984, karate teacher Jeffrey Doucet abducted 11-year-old Jody Plauché, driving him from Louisiana to a motel in California.
There, the paedophile continually raped and abused Jody, and he was only caught after allowing the child to make a collect call to his mum.
Jody was returned to his family and on 16 March Doucet, 25, was flown into Baton Rouge Airport in Louisiana, where he was escorted by police.
News crews were waiting for Doucet - as was Jody's dad, Gary, who was lurking by a telephone wearing a baseball cap and sunglasses.
As viewers watched on at home, Gary stepped out and shot Doucet in the head from three feet away.
As he was pinned by police, one officer pleaded: "Why, Gary, why?"
Those words stuck with Jody - who watched footage of the killing 'over and over' - and they eventually formed the title of his memoir.
Doucet fell into a coma and died the next day, and Gary spent a weekend in prison over the killing, but ended up getting a suspended sentence for manslaughter, as well as probation and community service.
Gary was widely supported by the American public over the revenge killing, but Jody felt differently about his actions.
Speaking to The Sun in October, he said: "At first I was upset with what my father did because at age 11 - I just wanted Jeff to stop and not necessarily dead.
"I think for a lot of people who have not been satisfied by the American justice system my dad stands as a symbol of justice.
"My dad did what everybody says what they would do yet only few have done it. Plus, he didn't go to jail.
"That said, I can not and will condone his behaviour. I understand why he did what he did, but it is more important for a parent to be there to help support their child than put themselves in a place to be prosecuted."
Indeed, his actions drove a wedge between the pair, and Gary insisted he had no regrets about killing Doucet.
Jody said: "He told me he figured he was gonna die so he felt he had nothing to lose. He told me one of them, if not both of them were gonna die that night."
He continued: "It took a few months before things got back to normal. As far as regret, of course he regrets the whole thing happened but he never regretted his action.
"Being Catholic I believe having taken a human life, his first thoughts were that he was going to hell... and he was OK with that.
"But the local priest washed his feet and gave his absolution and forgiveness from God so I think that helped him mentally deal with the event."
Gary died from a stroke in 2014 and Jody hopes his book will help other victims of abuse.
He said: "The most important thing is to seek out the proper support. With the proper support you can work through it.
"Unfortunately, most people's support systems are friends and family... and most of them are terrible at providing the emotional support a sexual abuse victim faces."
Featured Image Credit: WBRZ
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