To many across the US and even around the world who followed its aftermath, the story of the 2013 Mother’s Day shooting in New Orleans – which injured 20 people at one of the city’s vaunted second-line parades – is a simple one.

Siblings who dealt drugs and were locked in a feud over territory indiscriminately fired into a crowd, mortally wounding one local writer and cultural advocate – Deborah “Big Red” Cotton – who died four years later.

Yet another of the shooting’s victims, the Nation’s environment correspondent, Mark Hertsgaard, uncovered an infinitely more layered reality when he re-explored the case, including the teenager whom it ultimately sent to prison for the rest of his life: Akein Scott. And a seminal moment of that reality dates back to when Scott was beaten nearly to death when he was one year old by his mother’s boyfriend, who was then fatally shot in retaliation by Akein’s older siblings, as Hertsgaard recounts in his new book Big Red’s Mercy: The Shooting of Deborah Cotton and a Story of Race in America.

Hertsgaard, in a recent interview with the Guardian, spoke plainly about how there was limited empathy from some of his fellow victims – as well as the authorities who prosecuted Akein and his older brother Shawn Scott in connection with the Mother’s Day shooting – over the traumatic childhood episode the author uncovered. And Herstgaard, who was shot in the leg that day, said he did not judge them.

But, echoing a viewpoint he attributes to Cotton, who famously offered empathy to Akein Scott in public while condemning how the US’s revolving-door criminal withheld him from rehabilitation or opportunity, Hertsgaad remarked: “You have to see where things came from. You have to see it in context. And until we do see it in context, in that fuller context, we’re never going to fix it.”

Akein was two months away from turning two, living with his mother and her boyfriend – both of whom had fallen prey to the US’s crack epidemic – when he couldn’t stop crying one night. While his mother, Gladys, was out, her boyfriend became enraged at the toddler’s incessant sobs, so he struck the baby repeatedly.

An aunt of Akein recalled how the boyfriend, Kenneth Allen, hung Akein up in a sling carrier off the back of a door “like you would hang up a dress, and he hit that baby with a stick so bad, he broke his ribs and arm”, according to Hertsgaard.

Gladys Scott took her child to the hospital after she returned and found him shrieking for no obvious reason. The hospital enveloped the baby in a full body cast after X-raying and diagnosing him. Meanwhile, police arrested the mother on the spot for child endangerment.

Things only worsened from there. Two of Akein’s siblings had witnessed Allen brutalize him, and they told two of their older brothers who lived across town. Craig Scott, 18, and Michael Scott, 20, immediately raced over to the scene of the beating, traded angry words with Allen, shot him in the head and killed him.

The Scotts ended up with life sentences at Louisiana’s notorious state penitentiary, colloquially known as Angola. Akein and the rest of Gladys Scott’s children were dispersed among various other family members.

It is not an exaggeration to say that Akein carried the scars of the beating at the shooting he acknowledged carrying out on Sunday, 12 May 2013, at a second-line parade honoring Mother’s Day. In an internationally broadcast surveillance camera image that depicted him using his right hand to fire a gun into a crowd as the parade passed through the city’s seventh ward neighborhood, the then 19-year-old’s outstretched left forearm is bent in an unnatural angle.

“That twist was because baby Akein’s broken left arm had never healed properly after he left the hospital, and the family lacked the money to get the problem corrected,” Hertsgaard wrote in Big Red’s Mercy, which Pegasus Publishers released on 7 May.

Eventually, Akein and Shawn pleaded guilty to their roles in the Mother’s Day shooting, which they said targeted a particular rival and was meant to raise the profile of a heroin-dealing gang led by another of their brothers.

Beside Akein’s life term, which at age 30 he is serving at a federal prison in Texas, Shawn received 40 years in prison. Shawn Scott is being housed at a federal prison in central Louisiana, and the 35-year-old is scheduled to get out in June 2047.

The Scotts didn’t only victimize strangers, said Hertsgaard, who was visiting from out of town and befriended Cotton after they were both struck in a mass shooting that drew headlines across most of the English-speaking world.

Without realizing, the Scotts also injured their own nephew, 10 at the time, in what was the second time the boy had been shot during his brief life, as Hertsgaard noted in one of the many almost incomprehensible circumstances documented in his book.

Hertsgaard’s book takes pains to portray those facts as a collective failure of the environment which produced the Scotts – not just something personal to them. It links their fate to the city’s history of once being the US’s largest slave market – and the country’s inability to either confront or reconcile with that past, according to Hertsgaard and Cotton.

To Hertsgaard, one of the federal law enforcement agents who investigated the Mother’s Day shooting – and, coincidentally, grew up on the grounds of Louisiana’s Angola prison – perhaps put it best when informed of the life-altering beating Akein endured so early in his life.

“When you dig deep into someone like … Akein Scott, and you see how they grew up, you kind of get it how they came out the way they did,” Joe Frank of the US Bureau of Alcohol, Tobacco, Firearms and Explosives said to Hertsgaard. “If you grow up around violence from the time you first walk and talk, what are you going to do when you’re grown up?”