A sick Long Island music teacher plotted for nearly a decade to kill and rape his college coed sister-in-law – and even practiced the chokehold he would use, prosecutors said Wednesday.

Joseph Horner, 27, repeatedly broke into the downstairs apartment of 25-year-old Victoria Castle to check out “explicit photos” of his wife’s sister before carrying out the gruesome June 28 attack, Nassau County prosecutors said as they unsealed a murder indictment against Horner.

“He followed through on his plan that he admits he had been planning since 2017,” Assistant District Attorney Alexander DePalo said in court. “He is seen on video practicing the chokehold that he would place her in.

“He placed her in the chokehold, removed her clothing and raped the victim until he completed,” DePalo said. “Then he waited for the police on the front doorstep, calm and collected.”

Horner’s wife, who had been away on a bachelorette trip at the time, sat in court sobbing Wednesday, comforted by her parents as her husband pleaded not guilty.

The accused killer, a music teacher at the Oceanside School District, was ordered held without bail by Nassau County Judge Howard Sturim.

Castle, a PhD student at Stony Brook University, lived downstairs from Horner and her sister in the North Oak Street home in North Massapequa.

Prosecutors said the accused killer lured her to her death by asking her to help him move a keyboard – but instead ambushed her from behind and allegedly carried out the attack.

Just 15 minutes before the alleged attack, Horner practiced the chokehold on a stuffed animal – and even took a FaceTime call from his wife after rendering her unconscious, acting as if everything was normal before he proceeded to rape his sister-in-law, prosecutors said.

“He answered like nothing was wrong,” Nassau County DA Anne Donnelly said. “He acted like he hadn’t just left her sister lying on a bed.”

Horner is charged with first-degree and second-degree murder, and two counts for first-degree rape, and faces life in prison without parole if convicted.