Malcolm Campbell, from Rugeley, made more than 400 arrests while working with the police in towns and cities across the West Midlands.
But – after being assaulted on his third day at work in his role as retail security officer – he went through a "fanatical" obsession with stopping crime, between 2002 to 2006.
And now the former Lance Corporal has shared his story through his book Cool Sign Charley One – which he says is a "compelling" true story.
Mr Campbell, who now lives in Kidderminster, said: "I never thought about writing a book, but it's funny how life goes sometimes.
"I'm an ex-Lance Corporal with the British forces and when I left I decided to work alongside the police as a security guard, stopping town and city centre criminals.
"On my third day in the role I was terribly assaulted by a criminal who was high on crack cocaine – and that assault caused me to be withdrawn. I was in a right mess for four weeks.
"Then I was called to an incident where an old lady had been mugged and beaten in the street and I remember holding her in my arms and I forgot about my own assault. It stirred the soldier in me.
"I was successful in detaining and arresting the culprit and over a four year period I made 422 arrests – the book is about my work situation and a bit of my personal life."
Mr Campbell said it was a "very dark journey" over four years of his life where his family and friends feared the worst – with his social life going "down the toilet".
He later became involved with the "darker side" of society and described the experience as "shocking" as he learned why people turn to crime.
"It's a journey that I go full circle in – I started with hating the people who do those things and at thee end, I understand them," he said.
"I detained an Iraqi veteran and I found out later he was a soldier like I was and it shone a great light on society as a whole.
"And I don't really put myself in a very good light through most of it – but it's true," he said.
The now-50-year-old joined the forces in 1996, leaving in 2000, and initially joined as a chef specialising in French cuisine – but later became involved with the training side.
He served alongside the 4 Regiment Army Air Corps at Wattisham Airfield in Suffolk and became a Lance Corporal – paid at a higher rank of corporal – within three years, instead of the usual six.
The book can be bought on Amazon or through Malcolm Campbell's website.