Kay was screaming when she told the 911 operator the 19-year-old Henry Wachtel was having a seizure when he started beating her, a police source said.
Cops rushed to the Hell’s Kitchen building on W. 55th Street at Eighth Avenue just before 9:30 a.m., police said, and found a badly beaten Kay face-up and unconscious on the living room floor.
Neighbors said they were alerted to the horror by shouting.
His clothes were being analyzed as he was being questioned at the Midtown North Precinct stationhouse Tuesday evening, the sources added.
Kay was taken with severe trauma -- including a fractured skull and eye socket and broken ribs - to Cornell Hospital where she died at 1:52 p.m., police sources said.
There was no indication Wachtel used a weapon in the deadly attack, police said.
“The police took him in handcuffs. He kept apologizing to his mother the whole time - he kept saying, ‘I didn’t mean it,” neighbor Jonathan Cohen, 49, said the building’s doorman told him.
“He was extremely upset, wailing and crying,” said another neighbor. “When they took Karyn out on a stretcher, she wasn’t moving and her face was covered up with cloth,” he added.
Kay, who works as a teacher at La Guardia High School and is listed as a visiting instructor at Pratt Institute in Brooklyn, lived with her son for more than five years wrote and produced “Call Me”, a feature film that won awards for screenwriting, a Pratt Institute spokesman said.
"No matter how many times she saw them (“The Three Stooges”), she'd laugh out loud," he said.
When teaching poetry, one of her favorites was Langston Hughes.
"She used to talk about her son a lot. She'd say she had a son our age and she understood why we behaved the way we did, moving around and not listening."
Rodriguez said he'd remember the petite, conservatively dressed woman as warm person who liked to hug her students and spread good cheer.
"She was really happy all the time," he said.
Wachtel starred in a short film called “Our Time,” where he’s seen bizarrely shaving off his hair in the movie’s trailer.
“The Henry I know is a great guy and he’s an amazing actor,” said director of “Our Time” Tatianna Kantorowicz, who declined to comment any further about Wachtel.