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 as 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 inside the parking lot, where local boaters and club members like Millwater were known to park, officials said.

Kacy Corso was charged with second-degree murder in the violent death of Christopher Millwater on Nov. 10, according to Suffolk County police. Facebook/Kacy Corso

Kacy Corso is pictured during her arraignment. Pool

Corso was picked up by authorities when she returned to Long island Wednesday, prosecutors said. Pool

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 down on the ground, prosecutors said.

The new boyfriend then 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 Wednesday, prosecutors said.

Corso appeared in court in pajama pants and a hoodie as Millwater’s family watched the proceedings. Pool

Millwater was found gravely injured outside of the Stony Brook Yacht Club around 3:40 a.m. Facebook/Christopher Millwater

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 was murdered outside of the Stony Brook Yacht Club. Dennis A. Clark

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.

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

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

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

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.