Mob justice is best served cold.

A crooked Nassau County police detective moonlighting as a Bonanno soldier helped fuel a mini-mob war on Long Island — choosing “the crime family over the public he swore to protect,” a court heard Tuesday.

Ex-Det. Hector Rosario conspired to target rival Genovese mafiosos in the feud that spilled out after the organized crime clans struck an unusual agreement to split the proceeds of a gelato shop’s backroom gambling den, according to the feds.

But peace didn’t last and Rosario “sold himself” to the Bonnanos — even staging a fake police raid at a covert casino run by the Genoveses, Brooklyn federal prosecutors said in opening remarks at the alleged dirty cop’s trial.

Hector Rosario was a former Nassau County detective who reportedly helped tip off the Bonannos mafia in Long Island, including staging fake police raids. (Howard Schnapp/Newsday RM)

“For Hector to go in there, to Sal’s Shoe Repairs, and intimidate them in hopes they close down.”

Sal’s Shoe Repair had a card table in the back room with three or four gambling machines, Zummo said.

The fake raid of the shoe repair gambling den didn’t result in any arrests or citations, prosecutors said.

Zummo said that Rosario carried out an aborted raid at a coffee shop with a gambling den in Valley Stream owned by the Gambino family, because it was less than a mile from “Soccer Club,” a Bonanno-operated parlor owned by Sal Russo.

But the would-be raid went south when Rosario and a crew of a few guys weren’t able to get inside the gambling den, Zummo said.

The reason: it had a buzzer and they weren’t allowed inside, Zummo testified.

Rosario liked “again, and again” when pressed by the FBI during interviews in 2020, claiming he didn’t know anything about gambling dens, prosecutors said.

He also pulled information of a rival mob member from a law enforcement database and gave it to the Bonanno family, according to prosecutors.

The Bonanno family paid Rosario $1,500 per month during the time of the raids, which went on for a “few months,” according to Zummo.

Zummo testified that Rosario also tipped him that he was under investigation and to “stay off the phone” because “the feds are listening.”

He testified that he considered Rosario to be a “street guy.”

“Street guy means he’d break the law if he had to,” he said.

Zummo was arrested in 2017 alongside Russo in a drug-trafficking scheme that involved a $40,000 sale of cocaine in a Manhattan gelato shop.

Both are cooperating with the feds in the case.

Rosario’s defense attorney Lou Freeman told jurors that the former cop had made a false statement to authorities, but it was about a marijuana grow house in Queens that wasn’t material to the case.

He also argued that witnesses who are convicted of racketeering, distribution of cocaine and other serious crimes would be testifying against Rosario, who has pleaded not guilty and is out on bail.

“Each of these witnesses were in organized crime,” he said.

Sal Russo, who was once a “close friend” of Rosario’s, had put the former Nassau cop on the feds’ radar in hopes for a more lenient sentence, Freeman said.

“You will hear Sal Russo made up information about Hector Rosario to get one more notch on his belt because more notches on a cooperating witness’s belt mean less jail time,” Freeman said.

The feds confirmed they will be calling three cooperating witnesses during the trial, which resumes Wednesday.