He got out on good behavior.

Then Ramon Rivera, a troubled, homeless man with a history of arrests and mental problems, went on a vicious daytime stabbing spree across Manhattan on Monday, killing three people at random using a pair of large kitchen knives, authorities said.

Almost immediately, Rivera became every New Yorker’s nightmare materialized in flesh and bone. His case represented two major criticisms of the city’s criminal justice system — revolving-door justice and a lack of treatment for the mentally ill.

The failures would come under a harsh spotlight twice more this week.

On Tuesday night in Queens, convicted killer Gary Worthy, who was out on the street despite being on lifetime parole for decades of offenses, shot and wounded a police officer before he was fatally shot in the face during a wild shootout on a busy Jamaica street.

The next morning, 25-year-old Joshua Zinberg targeted a Danish tourist, slashing his victim in the face on the Upper West Side for “f——g with someone’s paranoia,” Zinberg said at his arraignment. Police and prosecutors described it as an “unprovoked” attack.

When Rivera, 54, was released from jail in October, officials acknowledged that he had a history of serious mental illness. Less than a year earlier, he called 911 on himself, telling authorities that he “felt suicidal and homicidal,” police sources said.

After a series of non-violent arrests, including a burglary at a Second Ave. convenience store, Rivera found himself in jail facing a 364-day sentence on Rikers Island. From there, he was sent twice to the Bellevue psychiatric unit, where on Sept. 4, he allegedly assaulted a correction officer.

At a hearing, he pleaded guilty in exchange for a 90-day sentence. The sentence was ordered to run concurrently, so his freedom was still less than a year away.

On Oct. 17, he was released from Rikers after serving two-thirds of his sentence.

Officially, he was released “due to time served,” a city Department of Correction spokeswoman said. But those in law enforcement circles know it as “getting out on good behavior” or “good time” — despite the attack on a Correction officer four weeks earlier.

“That’s the million dollar question, why was he allowed out?” a law enforcement source with knowledge of the case said. “He should never have been released early, especially after the attack at Bellevue.”

But there was no choice in the matter: if someone is being held in New York City, early release is commonplace and rarely scrutinized.

“It’s an automatic deduction applied to each sentence if they’re being held in the city,” a retired Department of Correction official confirmed. “It’s not considered if the person is deserving or not.”

After Rivera’s arrest Monday, Mayor Eric Adams called his case “a clear example of a criminal justice system and mental health system that continues to fail New Yorkers.”

“There’s some real questions that we need to look at on why he was on the street and he has some severe mental health issues that should have been examined,” he said.

The attacks were all random and unprovoked — adding to city dwellers’ worst fear that the ambushes could happen to anyone.

“He just walked up to them and attacked them with the knives,” said NYPD Chief of Detectives Joseph Kenny.

The first victim, construction worker Angel Lata Landi, was stabbed about 8:20 a.m. on W. 19th St. near Tenth Ave. in Chelsea, police said. The suspect walked up to the construction worker and began plunging his knife into the victim before running off, cops said.

“He’s standing in front of a construction site where he is employed when without warning he is stabbed in the abdomen,” Kenny said.

The second victim, a 68-year-old man, was fishing in the East River Promenade near the Water Club when he was stabbed at 10:27 a.m. at E. 30th St. and the FDR Drive in Kips Bay, said police.

Kenny said that victim was stabbed “multiple times in the body.”

Less than a half-hour later, at 10:55 a.m. a 36-year-old woman was stabbed at E. 42nd St. and First Ave. in Murray Hill.

Police ultimately caught up with the suspect, and stopped his trail of horror on E. 46th St. and First Ave.

Two of the victims died soon after being transported to the hospital, NYPD Chief of Detectives Joe Kenny said. A third died later Monday evening, police said.

Two bloody carving knives were recovered from the suspect, whose clothing was also drenched in blood, Kenny said.

According to sources, Rivera confessed to the killings in custody.

“Today we have three innocent New Yorkers going about their lives, the victims of terrible, terrible assaults,” Adams said, adding the brutal attacks “left us searching for answers how something like this could happen.”

“There’s a real question as to why he was on the street,” said Adams of Rivera. “He has some serious mental health issues.”

Rivera’s criminal history includes arrests in New York, New Jersey, Ohio and Florida.

Even after Rivera was released in October, he was quickly arrested again and charged with stealing a bowl priced at nearly $1,500 from a ritzy Jonathan Adler store on West Broadway last year.

The Manhattan DA’s office asked for bail, but Rivera was released.

A similar pattern was seen with Worthy, a 57-year-old career criminal known as “Green Eyes” who was gunned down by an NYPD officer after he held up two stores at gunpoint, cops said. A bystander, a 26-year-old woman, was wounded in the shootout.

After two separate prison sentences that put him behind bars for a total of 26 years, Worthy was released in November 2021 and put on lifetime parole.

He had been arrested seven times since his release three years ago, mostly for drug possession, burglary, and assault, but his parole was never violated and he was never brought back to prison.

After an arrest in August, the Queens District Attorney’s office asked he be held on $120,000 bail based on his criminal record and past convictions. Judge Edward Daniels disagreed and released him without bail. State parole asked that Worthy be held so parole violation proceedings could begin, but Edwards gave him supervised release, putting him in a program that assured his return to court.

“The judge has the sole authority to make the decision whether to remand or release on recognizance,” a spokesman for the state Department of Corrections and Community Supervision said at the time.

According to court records Edwards noted at the arraignment that Worthy had always been in contact with his parole officers and his release didn’t “present a substantial risk of failing to appear” to his parole revocation hearings.

Before his arrest on Thursday, Zinberg, 25, had never been in trouble with the law, but his story was just as tragic. After threatening co-workers and worrying his family, he selected a complete stranger on Columbus Ave. to bloody his blade.

His victim happened to be a retired professional soccer player visiting the U.S. from Denmark, cops said.

The troubled young man — who had been in a car — crept up to the tourist from behind and slashed him across the right side of his face “through the victim’s ear and into the victim’s neck with what appeared to be a knife,” Manhattan Assistant District Attorney Alexander Oltarsh explained as Zinberg was arraigned on attempted murder charges.

“This attack was entirely unprovoked,” Oltarsh said. “But it is clear that the defendant deliberately targeted the victim.”

Zinberg was arrested after his mother turned him in, and he was ordered held without bail.

He’s currently sitting on Rikers Island — right where Rivera was before he got out on good behavior.

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(With Graham Rayman.)

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