Four years ago, Lillyth Quillan cowered behind a padlocked door as her teenage son, taller and stronger than she is, paced back and forth in a rage.

Suddenly he went quiet. “Don’t let me hurt you, Mom,” he said, his voice sounding chillingly calm.

It was the first time the high school freshman had used that particular tone, but he continued to deploy it as he menaced his mom and dad.

“He used the kind of language of abusive husbands — manipulating and controlling,” says Quillan, who had installed locks on every door in her house except her son’s bedroom. “I was terrified of what he was going to do next.”

The boy — whom Quillan chooses to call Kevin in her interview with The Post in reference to the unnerving Lionel Shriver novel “We Need To Talk About Kevin” about a school shooter in upstate New York — was out of control.

After years of cruel and violent behavior plus multiple suspensions and expulsions from school, psychiatrists finally diagnosed the then-14-year-old Kevin with “conduct disorder,” which, in its most extreme form, can be a precursor to psychopathy.

Psychopathy, which is often used interchangeably with the term sociopathy, is believed to affect 1 percent of adults. Key attributes that sociopaths and psychopaths have in common include a disregard for laws, social mores and the rights of others, a failure to feel remorse or guilt and, in some but not all cases, a tendency to violence.

The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) dictates that people under the age of 18 cannot be labelled psychopaths. However, in 2013 the American Psychiatric Association decided to include the condition “conduct disorder with callous and unemotional traits” for children ages 12 and over.

According to a 2001 report published in the journal American Family Physician, approximately 6 to 16 percent of boys and 2 to 9 percent of girls meet the diagnostic criteria for conduct disorder — only a fraction of which have the “callous and unemotional” label that can potentially lead to psychopathy in adulthood.

More than 50 studies have found that kids with the latter diagnosis are more likely to become criminals or display aggressive, psychopathic traits later in life. It has been reported that Nikolas Cruz, the 19-year-old who allegedly shot and killed 17 people at Marjory Stoneman Douglas High School in Parkland, Fla., last month showed classic signs of the disorder as a child, including abusing animals.

“Psychopaths don’t just appear when they are 20. They are always different from an early age,” Kent Kiehl, a psychology professor at the University of New Mexico and the author of “The Psychopath Whisperer,” tells The Post.

Characteristics to look for — as detailed in the widely used Hare Psychopathy Checklist Youth Version considered by clinicians and researchers to be the “gold standard” in assessing psychopathy — include lack of empathy, lack of guilt and regret, pathological lying, grandiose self-worth and failure to accept responsibility for actions such as fighting and bullying.

“Individuals who score high on those traits are more likely to produce further violence,” adds Kiehl. “If they are sanctioned but continue on the same path, it’s not a perfect indicator, but it’s enough to cause concern.”

Kiehl notes that research has shown that psychopathy is hereditary roughly half of the time. But his own breakthrough was the discovery that the psychopathic brain has a different structure than a “normal” one.

In 2014, he conducted a major study that found at least two abnormalities in the brains of adult psychopaths. There was a lack of gray matter in the section involved in processing emotions, while the area that reacts to excitement and thrills is overactive. Related research showed the pattern is also occurred in the brains of “callous and unemotional” children. “Brain science has helped us understand what is different about these kids,” adds Kiehl.

At the moment, there is no such thing as a “cure” for psychopathy or conduct disorder. But early intervention can be key for harm reduction, even with children as young as 2 or 3.

Paul Frick, a psychology professor at Louisiana State University and the author of “Conduct Disorder and Severe Antisocial Behavior,” recommends a range of therapies, most of which revolve around rewards systems rather than punishments.

“There are so-called ‘emotion coaching’ techniques that parents and therapists can employ to help children pay attention to the feelings of others,” he explains. “We find that they miss the cues that another child is upset.

“By saying: ‘Can you see how Johnny is feeling?’ [when a toy is snatched from him] and getting them to respond correctly, you can motivate them. You give them a star or a sticker as an incentive.

“Even though it doesn’t come naturally to them, they can learn others’ perspectives.”

Experts can identify a callous and unemotional child when they are as young as 3 or 4. Faced with a crying peer, typically developing children either try to comfort them or take flight. But those with the mental condition remain in place, showing apathy and coldness.

Remarkably, the psychology department at King’s College London has been able to trace the characteristics back to infancy. They tested more than 200 babies at 5 months old, tracking whether they preferred looking at a person’s face or at a red ball. The tots who favored the ball displayed more callous traits two and a half years later.

For Quillan, hindsight is 20/20, but she distinctly recalls the first signs that Kevin had behavioral issues at the age of just 8 months.

“He had teeth and would bite me while he was breast-feeding and he would laugh. He thought it was hilarious. I tried looking very sad and mimicking crying to show it was hurting me, but he would only laugh,” says Quillan, who ended up having to put him on formula.

“It didn’t occur to me until much later that this was a child for whom the amusement of my reaction when he bit me was a greater reward than food.”

‘Psychopaths don’t just appear when they are 20. They are always different from an early age.’

Now 18, Kevin, who has had numerous run-ins with police, including for shoplifting, was made a ward of state and no longer lives with his parents. He lives in a residential school for “at-risk” youth in California, where he is on a waiting list to receive treatment, such as therapy, to build empathy.

“Because there is no real treatment for conduct disorder. All you can do is wait for your child to be arrested and enter the juvenile system and hope they get better,” says his 40-year-old homemaker mom.

“Luckily, Kevin is no longer violent and is actually cooperative.”

He is doing so well that he is about to receive his high school diploma, recently won an award for wrestling and has encouraged his mother to tell his story.

Now Quillian, who has no other kids, is focusing on advocacy and encouraging parents facing similar nightmares to hers. Three years ago, she formed a support group for families with kids with CD that has 420 members worldwide. More recently, she launched the Society for Treatment Options for Potential Psychopaths to bring awareness and to campaign for treatment for these children before they cause serious harm.

Adds Quillan: “As every news article came out about Parkland and Nikolas Cruz, I thought: ‘My God, this could easily be one of our kids.’”

— Additional reporting by Molly Shea