Charity Lee was approaching the supermarket checkout when a fellow shopper blocked her path.

“I know who you are,” shouted the woman. “You should watch your son be executed, because he is a monster and you raised him!”

Drawing on all the strength she could muster, Lee shakily replied: “Ma’am. I don’t know who you are, but you really need to get counseling.”

With that, the mother of child-killer Paris Bennett, age 13, abandoned her grocery cart and rushed out.

The stranger’s harsh judgment only added to her agony: In a shocking twist, her son’s victim was Lee’s 4-year-old daughter, Ella.

Paris had stabbed his half-sister to death in cold blood with a kitchen knife.

“There have been other moments like that,” Lee tells The Post of the 2007 incident in the Abilene, Texas, store. “But people often have one opinion at first, and then change it once they’ve talked to me and offer compassion.”

The 44-year-old’s powerful story of grief, love, fear and forgiveness is featured in the documentary “The Family I Had,” airing on Investigation Discovery at 9 p.m. Thursday.

It examines Lee’s conflicted emotions as she struggles to come to terms with the catastrophe that tore her life apart 10 years ago.

‘I have forgiven Paris for what he did, but it’s an ongoing process.’

A prison rights activist, she keeps Ella’s memory alive while frequently visiting her now-24-year-old son in jail. He is serving a 40-year sentence (the maximum in Texas for a juvenile for murder) and will be eligible for parole in 2027.

“I have forgiven Paris for what he did, but it’s an ongoing process,” explains Lee. “If he was free [from captivity], I would be frightened of him.

“The fact that he is incarcerated gives me peace of mind, but I worry about his own safety.”

It was about 12:30 a.m. on Feb. 5, 2007, when Lee was met by cops at a Buffalo Wild Wings near Abilene where she worked as a waitress.

“[The police] told me that my daughter had been hurt,” she recalls in the film. “And I was saying, ‘You need to take me to Ella now,’ and they were like, ‘You can’t go … she’s dead.’

“And that made no sense, because I knew that I’d left her at home with a baby sitter and her brother, so I said, ‘Is my son OK?’ And they said, ‘We have him.’ … That’s when everything stopped making sense.”

Sometime around 10 p.m., Paris, an unusually gifted child with an IQ of 141, had convinced the baby sitter she could go home. It was after that, according to detectives, that he grabbed a knife and entered the room where Ella was sleeping.

He proceeded to beat and choke his little sister and stab her 17 times.

Next, the boy spent six minutes calling a school friend before waiting two minutes and phoning 911 to report the murder.

“He pretended to follow the dispatcher’s directions and do CPR,” says Lee. “But that was all a lie.” Cops found no evidence of attempted first aid.

At first, Paris claimed he suffered a vivid hallucination in which an inflamed, demonic version of Ella was laughing maniacally at him. But he later described how he had awakened that morning wanting to kill someone.

Says Lee: “Ella was an easy target — predators don’t ever pick on anybody bigger than themselves.”

Paris told investigators that his original plan was to murder Ella before lying in wait for his mother and stabbing her when she returned from work. “He said the first reason he didn’t go ahead with it was because it was a lot harder to kill someone than he thought,” says Lee. “The second reason was the realization if he’d killed me, I only would have suffered for five, 10, 15 minutes. But, if he left me alive [without Ella], I would suffer for the rest of my life.”

One of Paris’ motives was punishing his mother. A former heroin addict who kicked drugs shortly before conceiving her son, she’d briefly relapsed on cocaine when he was 12 and Ella was 3. This followed more than a decade of sobriety for Lee. Paris claimed she had put her habit before him.

“The only regret I’ve ever had about my own personal behavior is my relapse,” says Lee, who has not abused narcotics since Ella’s death. “The fact is, it made him angry and he chose to handle it that way [by killing Ella].

“It’s just another indication that he is a sociopath. I don’t regret how I’ve handled Paris, I feel like I’ve been true to myself and followed the right path.”

After his sentencing, an assessor told Lee she deserved to know that her son was a sociopath. Psychiatrists whom she hired when Paris was 15 agreed that, had he been 18 and old enough ​to qualify ​for the label, they would have diagnosed him as having anti-social personality disorder​ (sociopath​y​)​.​ He confessed to having had homicidal thoughts since the age of 8, often expressing them through violent and disturbing drawings.​

Since becoming an adult, Paris has refused further psychological evaluations. In a jailhouse interview behind glass in “The Family I Had,” he tells directors Carlye Rubin and Katie Green: “I chose to do my crime and I take full responsibility for my crime. And I wouldn’t say there was a predisposition to what happened.

“I’m not insane and I don’t suffer from any mental illness.”

‘I am not going to be that parent who abandons their kid.’

While Lee describes him as “manipulative” and “narcissistic,” she is quick to explain how her maternal instinct means she puts her love for her son above her anger.

“I sometimes have to say to myself [during visits]: ‘Okay, Charity, take a breath, you know how Paris is wired,’ ” she says. “But I am not going to be that parent who abandons their kid.”

In 2012, the single stay-at-home mom became pregnant with a third child, Phoenix, now 4. She was delighted to at last be given a new hope.

The pair now lives in Savannah, Ga., and Lee goes to see Paris in the Lone Star State for four-hour stretches as often as she can. She is grateful that prison rules ban him from having visitors under age 17. “Texas won’t allow him to see Phoenix because he killed a child.

“If Paris wasn’t in prison or was able to meet Phoenix, I would have to do a lot more soul-searching.”

Mercifully, the soul-searching is helping Lee cope with the devastating loss of Ella, whom she describes as “extroverted, very outspoken and very smart.”

Butterflies became Ella’s symbol after her preschool teachers gave her mother a remarkable picture she had painted of the insect. A friend also found a butterfly brooch in Lee’s back yard on the day she finally returned to the house where the murder took place.

Ever since then, Lee associates butterflies with the presence of her daughter.

“That’s her thing, I guess,” adds Lee, who has a collection of butterfly tattoos.

In 2011, the grieving mom founded the nonprofit ELLA Foundation — an acronym for Empathy, Love, Lessons and Action — which assists people involved in the criminal justice system and those affected by trauma.

“On the night that Ella died, I vowed to do something meaningful in her memory,” concludes Lee. “It also gave me a place to direct my rage, other than at my child.”

Through all this, she is also a loving mother to Phoenix, who, she says, gave her “joy, life and happiness again.”

“Because I was living with the dead, I was barely living,” adds Lee. “Phoenix really brought me back into the moment.”