Joseph Horner, of North Massapequa, charged with killing sister-in-law, practiced chokehold on stuffed animal before attack

 An obsessed Oceanside music teacher stalked his sister-in-law — practicing the chokehold he used on her moments before he attacked her — then raped her unconscious body and ultimately killed her, prosecutors said Wednesday morning at his arraignment in Nassau County Court.

Joseph Horner, 27, pleaded not guilty before acting Supreme Court Justice Howard Sturim in Mineola to three counts of murder and two counts of rape.

His attorney Mitchell Barnett declined to comment after the hearing.

Nassau County District Attorney Anne Donnelly said that Horner been fixated his sister-in-law, Victoria Castle, 25, who lived alone on the lower floor of a split-level North Massapequa home where Horner and his wife lived on North Oak Street.

He first met her a decade ago while he was a student at Ithaca College, where Castle would visit her sister.

Donnelly said that he saved Facebook photos of her on his phone and when she was not in her apartment, to which he had sneaked in to search her tablet for explicit photos of her.

He told detectives that he and his wife had a falling out two years ago when she found photos of her sister on his electronic device.

"Our investigation revealed that this defendant was infatuated with, and lusted after, the victim from the moment he met her in 2017," Donnelly said.

On June 29, while Horner’s wife was out of town on at a bachelorette party, according to Alex DePalo, an assistant district attorney, he was seen on video using a stuffed animal to practice the chokehold he would later use to render Castle unconscious, prosecutors said.

Shortly before 8 a.m., he asked Castle to help him move a keyboard that she had borrowed upstairs from her part of the house to his, investigators said.

As Castle went back down to her apartment to retrieve the keyboard stand, Horner came up behind her and put her in a chokehold that knocked her out, officials said.

Horner told police that he held his sister-in-law by the throat for three minutes, according to a transcript.

“I wanted to stop,” he said. “But at that point I knew that it was too like I was too far gone.”

At some point during the attack, Donnelly said, Horner left Castle on the bed and went upstairs to talk to his wife on a video chat app to assure her that their pet cats were OK.

"During that call, the defendant acted as if nothing was wrong, as if he hadn't just violently attacked her sister, as if Victoria's body wasn't laying motionless just a floor below," Donnelly said.

After the attack, Horner changed his clothes and called 911 to tell police what he had done, the prosecutor said.

When they arrived, he we was sitting on his front porch. “I did it,” the told the responding officers, prosecutors said. “I killed her.”

He later told detectives that “My intention was not to kill her ultimately,” he said. “Ultimately was more the sexual side of it. … I intended her to be unconscious and in the heat of the moment, I just … it got away from me.”

Castle was taken to St. Joseph's hospital where she was declared dead. An autopsy later determined strangulation was the cause of death.

Horner taught music at the Oceanside Road Elementary School No. 2, according to online records.

Castle was living below the couple while she attended Stony Brook University working toward a PhD in geology.

"Victoria was a promising young woman who had a bright future ahead of her," Donnelly said after the arraignment. "Her life was senselessly cut short by a man she had welcomed into her family with open arms."

After the detectives had read Horner his rights, he told them that he had become addicted to online gambling and spent $500 on internet pornography the night before the attack.

Horner said that he was struggling financially. He had taken out a short-term loan of $1,200 to feed his gambling habit. He also told police, according to the transcript, that he had lied to his wife and others that he had his master's degree and that he would soon be getting paid more because of the advanced degree.

"Last night, I felt like it was the end and to kind of cement it for myself, I spent close to $500 on OnlyFans 'cause I knew it was the end. I knew that I was going to be in trouble," according to a transcript of his interview with police. "I knew that my wife was going to find out anyway. And when I knew that, that's when I knew that I need to go away."