Police say the music teacher raped and then strangled his relative last month.
Disturbing new details were revealed in court on Long Island on Wednesday.
Prosecutors said Joseph Horner even practiced how he'd put the victim into a chokehold so she lost consciousness, and then sexually assaulted her.
He was a well-liked music teacher in the Oceanside School District, but no one knew about the alleged secret life of 27-year-old.
Horner was charged with both murder 1, murder 2, and two counts of rape.
Prosecutors released a chilling screen grab of a security video showing him practicing a chokehold on a stuffed animal just moments before they say, he used that chokehold to strangle his wife's sister, Victoria Castle.
"I think that just goes to the twisted, violent person that he is," said Anne Donnelly, Nassau County District Attorney.
Castle, 25, was getting her PhD at Stonybrook University. She had lived downstairs from Horner and his wife in a home in North Massapequa.
Horner's wife was in court Wednesday with her whole family. They made no statement to reporters, nor did Horner's defense attorney, but authorities say Horner strangled, then raped Castle, then calmly called 9-1-1.
"Changed his clothes, sat on the porch and waited for them," Donnelly said. "He basically, the way it was explained to me, had very little affect. 'I did it. She's downstairs. I'll wait here.'"
Horners' wife was out of town while he allegedly committed the crime, but what's even more disturbing is the DA also says he took a FaceTime call from her during the killing.
"He acted like nothing was wrong, he acted like he hadn't just left her sister, lying on a bed," Donnelly said.
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