Accused Gilgo Beach killer Rex Heuermann reveled in telling underlings how he stalked and butchered animals — and staked out a business rival to the point where freaked-out workers began locking themselves in, a new report says.

Mary Shell, who worked with the hulking architect for several years, wrote in New York magazine that Heuermann liked to hire petite women — fitting the description of his alleged victims — and once referred to a client of his Midtown office as a “target.”

Shell said coworkers “described how he gushed about his bear-hunting trips in particular: baiting an area and lying in wait. 

“More than once, he gleefully described the process of dressing the game, seeming to delight in grossing his employees out,” Shell wrote. “Once, at a job site, he spotted a client approaching and, speaking to my boss over the phone, said, ‘Target in sight.’ ”

Shell said she first met Heuermann in 2007 when her employer hired the accused killer to work on a project.

“Just a few months earlier, authorities say he murdered his first known victim, 25-year-old Maureen Brainard-Barnes,” the woman noted.

Shell recalled that when she met Heuermann at his office, “his face turned red like that of a shy teenage boy who had been surprised by a hot girl in the cafeteria.”

She said Heuermann, who founded RH Consultants and Associates in 1994, peddled himself to clients as a master at navigating the Big Apple’s maze of building codes and zoning restrictions.

“His office was mostly staffed by women like myself, young and petite, the girl-next-door type,” said Shell, who eventually accepted a job with Heuermann’s firm as a consultant. “We knew he was married with a family in Massapequa, but he never spoke of them.

“He famously loved guns, and we all knew that he was a collector,” she wrote. “He would frequently invite colleagues to go hunting or shooting at the range, including one of my attractive co-workers that he seemed to mention the idea to every time they spoke.

“She never accepted the offer, confiding in me it sounded dangerous and creepy.”

Shell said that in another particularly disturbing incident, Heuermann was furious when a female employee bolted to another firm, renting out space in her old employer’s office.

“This enraged him,” she said of the defection. “Knowing they kept late hours, he was spotted outside the building, gazing up into the second-floor windows in the early evening. My colleagues’ minds went quickly to his gun collection, and they began locking the elevator and the doors.”

According to police, this was during a time when Heuermann was allegedly soliciting escorts online. At least three of them were found dead along Gilgo Beach along the South Shore in 2010, cops say.

Heuermann, 59, is now charged with murder in the deaths of Melissa Barthelemy, Amber Lynn Costello and Megan Waterman and is the prime suspect in the slaying of Maureen Brainard-Barnes.

Shell said that when she heard about her old colleague’s arrest, she was nonetheless surprised.

“Even though his face was on the [social media] post, my immediate thought was how strange it is that there are two people named Rex Heuermann,” she said.

“Then it dawned on me. I called a former co-worker immediately and asked, ‘Is this real?’ ”