Three decades ago, he was the ultimate face of evil — a monster who beat his illegally adopted 6-year-daughter, Lisa Steinberg, into a coma, then smoked freebase cocaine until it was too late to save her.

The little girl’s fatal beating on Nov. 1, 1987, would make front-page headlines, prompt outcry over the state of the child welfare system and land Joel Steinberg nearly 17 years behind bars.

Today, Steinberg is living the quiet life of an aging loner in Harlem, hitting up strangers for cigarettes and Wi-Fi connections as he ekes out a living as a disbarred lawyer.

Most days, he shuffles around the corner to ask the neighborhood’s produce vendor for handouts of rotting fruit and vegetables to use as fertilizer on the garden outside his rear ground-floor apartment.

“You have to remember I’m a pariah, so it’s not that easy for me,” Steinberg, 76, said during an exclusive interview with The Post last week.

“If you go out there and put a picture of me in the paper, I can’t take a subway for two weeks because some fat person will decide to say, ‘I know you, and you’re a piece of s- -t.’ . . . And then I turn around, and he punches me in the f–king nose.”’

To New Yorkers who know about his heinous crime, Steinberg deserves far worse than a sock in the face.

On the 30th anniversary of Lisa’s fatal beating, he remains remorseless, steadfastly denying any guilt for ending her life.

“What did she die of? She died of pulling the plug,” he said, referring to the brain-dead child finally being taken off life support four days after he brutalized her.

Steinberg even disgustingly claims that he can’t come to grips with Lisa’s death because it’s just too painful — for him.

“One of the things I do, instead of suppressing the memory, I keep the memory alive,” he said.

“And what I notice, and what people have pointed out to me, I speak as if Lisa were alive. I don’t accept her loss. I don’t accept it as a loss. I accept it as a memory.”

Then he once again proves that his evil knows no bounds.

Asked if he had anything he wished he could say to Lisa, Steinberg answered in a cold tone that dripped with sarcasm: “Yeah, I’ll never kill you again, and I’ll never beat you up every day, and I’ll never make you a torture tot in a house of horror” — the last phrase something he also mentioned during a 1997 parole hearing.

Steinberg, a former Air Force lieutenant and graduate of New York University Law School, was 46 when he committed the vicious and heartless acts that turned him into one of the city’s most notorious bogeymen.

Around 6 p.m. that November evening, he was getting ready to head out for a business dinner when Lisa went into the bedroom of the Greenwich Village apartment where she lived with Steinberg, his girlfriend Hedda Nussbaum and another adopted child, a baby boy.

Lisa’s fatal mistake was having the audacity to ask her daddy if she could go with him.

Steinberg soon emerged carrying Lisa’s body into the bathroom and handing her to Nussbaum, then a 45-year-old former children’s book editor. Nussbaum laid the girl on the floor.

When Steinberg returned around 10 p.m., he had Nussbaum convert the cocaine left in their apartment into freebase and spent the next several hours smoking it.

At 6:33 a.m. the next day, Nussbaum called 911 and said Lisa wasn’t breathing, then opened the door to cops, who saw the woman had two black eyes, busted lips and a nose that was flattened and split down the middle.

Meanwhile, Lisa was comatose and filthy, her hair a matted, ratty mess and her feet covered in layers of grime.

The couple’s other illegally adopted child — Mitchell Steinberg, age 16 months — reeked of urine and was drinking from a bottle of spoiled milk while tied to a makeshift playpen in the family’s squalid, cluttered living room.

“Out of the whole apartment, the fish tank was the cleanest thing,” Joseph Petrizzo, a city welfare worker, would later testify in court.

Lisa was rushed to St. Vincent’s Hospital, where doctors found her covered in old and fresh bruises and her brain swollen from blunt trauma to her head. They quickly decided she was the victim of extensive abuse, and both Steinberg and Nussbaum were arrested on attempted murder charges.

The tragic girl was declared brain-dead Nov. 4 and removed from a ventilator at 8:40 a.m. Nov. 5. Her heart stopped beating 15 minutes later.

The charges against Steinberg and Nussbaum were quickly upgraded to murder, with Nussbaum eventually offering to testify against Steinberg, who went on trial in October 1988.

With no eyewitness to the blows that killed Lisa, prosecutors presented a circumstantial case based largely on the testimony of Nussbaum, against whom all charges were dropped.

During seven days on the witness stand that were televised live by local stations, Nussbaum tearfully described years of abuse at Steinberg’s hands, suggesting that she was a victim herself — of battered women’s syndrome.

She said Steinberg admitted to her that he had abused Lisa, saying he told her, “I knocked her down, and she didn’t want to get up again.”

For a motive, prosecutors pointed to the paranoia caused by Steinberg’s daily freebase habit.

The defense tried arguing that Nussbaum killed Lisa in a jealous rage, then pretended she was crazy to cut a deal and avoid punishment.

Jurors acquitted Steinberg of murder but convicted him of first-degree manslaughter, and he was sentenced to the maximum 8¹/₃ to 25 years in prison.

Despite being a near-model prisoner, he was denied parole five times, after repeatedly claiming his only role in Lisa’s death was failing to seek prompt medical care. He finally got mandatory parole June 30, 2004, then spent time in a Harlem halfway house and a Times Square hotel before landing in his apartment in 2006.

Steinberg spoke at length to The Post last week, wearing a shirt and jeans — and whining that he, too, is a victim, saying, “I’m not happy I lost the best years of my life.

“I listen to people today saying, ‘I don’t accept the death of my daughter.’ . . . And I understand that because the mind doesn’t let you.”

Steinberg also railed against the cops who busted him — griping that they trashed his apartment to make him look bad, including by overturning two of Lisa’s fish tanks.

“They dumped them over on the floor. Then they pulled all the books out. And then they took pictures . . . There were fish all over the floor,’’ he said. “I had a very expensive rug on the floor. They ruined it.

“You don’t get it. It’s really traumatic.”

He insisted that various medical reports proved that he hadn’t abused Lisa and accused doctors of failing to save her through improper treatment.

Steinberg also bizarrely tried to rewrite history by saying, “I wasn’t arrested. Hedda was.

“She was home, she was charged. And they knew — and there were people who said she behaved poorly [before]. I don’t know if that’s true; I was out of town,” he claimed.

Steinberg, who had more than $1 million in assets seized by the IRS following his arrest, said he later successfully sued and “got some of it back.”

But after reportedly claiming in 2006 to have been “working my ass off” as a laborer, he said he’s now beset by various health problems and making money by doing unspecified freelance legal work.

Under terms of a September court ruling, he owes more than $4 million to Lisa’s birth mom, Michele Launders, following lengthy litigation over a wrongful-death and negligence suit she filed against him in 1988.

Steinberg’s mood fluctuated wildly throughout the interview as he at times chatted amiably, smiled and laughed, only to work himself into a lather — his eyes flashing and beads of sweat forming on his brow — as he ranted about how unfairly he’s been portrayed in news reports.

At one point, the convicted killer made several threats over how his words would be reported by The Post.

“You’re putting me at risk, and if you put me at risk, be prepared to understand you have responsibility. And there are people who make judgments,” he said.

Twice during the interview, Steinberg strolled off to smoke cigarettes he bummed from strangers, and at one point upbraided a homeless woman who approached to beg for spare change.

“Not now, ma’am. Step off,” he commanded.

“This is a serious conversation.”

Afterward, he tried to justify his rude behavior.

“She didn’t show any lack of clarity, and any handicaps,” he reasoned. “I don’t want to be bothered and she doesn’t deserve the consideration.”

Many of Steinberg’s neighbors say they were unaware of the notorious ex-con in their midst until a visit from The Post, with one man expressing shock that no one had told him who lived downstairs.

“Damn, I don’t want to live in the same building as that guy,” the man said.

“I am not engaging with the kid-killer. I will look down if I see him. I’ve only been here for three weeks, but I ain’t letting my granddaughter visit. I am gonna buy pepper spray.”

A seventh-grade teacher at the nearby Columbia Secondary School was horrified to learn that Steinberg lived a block away.

“I remember being a teenager in Long Island and hearing about a wealthy lawyer killing a child,” she said. “I remember it being a really ugly story. I lived in a safe neighborhood, but after Steinberg made the news, the parents gave us curfews, and we had more restrictions.”

A man who lives across the street said he learned about Steinberg’s presence years ago but never told his wife for fear she’d want to move.

“He stays indoors and has a very distracted personality,” the man said. “He always walks as a hustler and always has an angle. He tries to interact, and I avoid eye contact.”

Two adult students who live in Steinberg’s building said he asked for their Wi-Fi passwords, with one of them, Fan Su, 29, calling it “the longest conversation we’ve had.”

“I’ve never seen him with friends,” the student said.

“I’ve heard rumors about him killing kids. I mind my own business and keep quiet. I don’t want to get mixed up with killers.”

Street vendor MD Rahman said Steinberg walks by his produce stand about once a day, “usually in the afternoons.”

Sometimes, Rahman said, Steinberg “dresses nicely . . . with a tie on. Other times, he looks shabby and unshaved.

“He will ask a lot for free food, old fruits and vegetable. I will give it to him if he is nice,” Rahman said.

“I hear he was the lawyer, he did something bad.”

Additional reporting by Julia Marsh and Stephanie Pagones