A “psychopath” art benefactor murdered beloved New York City artist Sabina Rosas — who was found dead in posh Hamptons spa — before shockingly shooting himself, her longtime love told The Post Wednesday.

Fashion designer Ryder Iwata said the suicide of Thomas Gannon, 56, likely forever deprived Rosas’ family and friends of answers of what happened inside a guest house at the $1,000-a-night Shou Sugi Ban House, where the 33-year-old artist was found murdered this week.

“Unfortunately, it’s a total miscarriage of justice that the man who killed her kind of got out of it,” Iwata said in an exclusive interview.

“It’s wicked and it’s cowardly.”

Police said Gannon died from a self-inflicted gunshot wound at his home in Honesdale, Pennsylvania.

Iwata, who has been in a relationship with Rosas since they met as Purchase College students in 2016, described her as the “greatest love” of his life. The pair planned to spend their lives together, he said.

“She would call me just her person, her greatest friend,” he said.

“That’s my person.”

The relationship between Rosas and Gannon, who owned a Majestic Tile, Granite and Flooring in Honesdale wasn’t romantic, Iwata said.

He said Gannon was an art benefactor to Rosas – and someone he only briefly met once, while moving furniture for his partner.

“I clocked that he was strange, and he turned out to be a psychopath,” Iwata said.