A man found murdered outside a swanky Long island yacht club was stabbed to death in an ambush attack by his ex-girlfriend’s new beau while his former lover egged him on, prosecutors said Thursday.

Delivery driver Kacy Corso, 33, and new boyfriend Victor Panebianco confronted Christopher Millwater in a parking lot near Stony Brook Yacht Club on Nov. 10 — with Panebianco demanding the victim say he and Corso were through as she told the killer “don’t believe him, don’t let him fool you,” the Suffolk County District Attorney’s Office said.

The crazed couple had allegedly rammed Millwater’s car and blocked the exit to trap him in the parking lot, where local boaters and club members like Millwater were known to park, officials said.

The deadly attack came after Corso slammed Millwater, 52, in a series of nutty social media posts where she warned him about contacting her and called him a “grave robber.”

Prosecutors said on the night of the attack, Corso plowed into Millwater’s vehicle, disabling it, before a knife-wielding Panebianco chased after him on foot — then pinned him on the ground.

The new boyfriend asked Millwater about his past relationship with Corso as she needled him, prosecutors said.

“Don’t listen to him, don’t let him fool you,” she allegedly told Panebianco before he stabbed Millwater multiple times in the heart and lungs, prosecutors said.

The new boyfriend asked Millwater about his past relationship with Corso before stabbing him multiple times in the heart and lungs, prosecutors said.

The pair fled to North Carolina following the attack, but Corso was picked up by authorities when she returned to Long Island on Wednesday, prosecutors said.

Corso appeared in court in pajama pants and a hoodie as Millwater’s family watched the proceedings, getting visibly emotional during prosecutors’ description of the attack.

Panebianco was arrested in North Carolina and is expected to be hauled back to New York to face the charges.

Both are facing second-degree murder charges. 

Millwater, of Selden, was found gravely injured around 3:40 a.m. and was taken to Stony Brook University Hospital, where he was pronounced dead, police said.

Corso apparently unleashed on Millwater and issued a chilling warning in a July 26 Facebook post.

“Whoever knows Christopher Millwater he is a grave robber… who [the f—k] steals a marker to a person’s final resting place because he cant accept NO for an answer or won’t accept I DONT WANT TO BE WITH HIM,” an account under the same name wrote.

“As you can see… psycho much?? If anyone knows him better talk some sense into him before it’s too late.”

Corso included screenshots that showed purported messages between her and Millwater.

She also posted a screenshot of another social media user criticizing Millwater.

Corso worked as an Uber and Doordash driver for the past eight months while living in North Carolina, according to prosecutors.

The murder shook the usually quiet North Shore suburb, with one resident previously telling News 12, “That usually doesn’t happen here.”

Millwater actively posted on Facebook, including one eerie online remark.

“Plenty of time to rest when I’m dead so now it’s a quick shower and gym time,” he wrote two days before he was killed.

Millwater’s family remembered the victim as a warm and giving person.

“He was really like a second father to me,” Millwater’s 17-year-old nephew Chase Millwater said outside the courthouse on Thursday.

“It’s upsetting to know people have the heart to do things like this to others,” the distraught relative added, describing his uncle as being “hunted” by the suspects. 

He said his uncle would take him fishing in the same area he was killed as the victim’s sister, Corinne Millwater, vowed she would “fight for my brother’s justice.”

She declined to talk about her sibling’s relationship with Corso.

“My brother was an outstanding man, he showed up for everyone. He didn’t have much, but whatever he had, he gave,” Corinne Millwater said.

Corso’s lawyer and her loved ones declined comment after court Thursday.