Princeton Man Found Not Guilty By Reason Of Insanity In Brother's Murder

MERCER COUNTY, NJ — A Princeton man who killed his younger brother with a knife and golf club while experiencing severe psychotic delusions has been found not guilty by reason of insanity.

A Mercer County judge ruled Wednesday that Matthew Hertgen was legally insane at the time of the killing — a conclusion that prosecutors themselves said they could not contest.

Matthew Hertgen, 32, was charged with first-degree murder, weapons offenses and animal cruelty in the death of his brother Joseph Hertgen, 27, at the family's Michelle Mews apartment in Princeton on Feb. 22, 2025.

At a bench hearing Wednesday, Judge Robert Lytle agreed with mental health experts from both sides that Matthew Hertgen was legally insane at the time of the killing.

"In his delusional state, the act was not wrong in the ordinary moral sense — it was required," Lytle said from the bench.

Mercer County Assistant Prosecutor Tim McCann did not fight the finding.

"It's the state's position the defendant has established the insanity defense by preponderance," McCann said. "It's my position the state concedes, and really can't contest that at this time."

Because of the not guilty by reason of insanity verdict, Matthew Hertgen will remain in custody until sentencing May 1, at which point he is expected to be committed to a psychiatric facility.

Dr. Gianni Pirelli, a clinical and forensic psychologist who evaluated Hertgen and reviewed his records, testified that Hertgen has schizophrenia and has struggled with severe mental illness since 2021.

"He believes that the spirit that overtook him in 2021 is basically too powerful for any medications or mental health treatment that would come about," Pirelli testified. "Anytime he closes his eyes, he's seeing tremendous visions."

Pirelli also told the court that Hertgen had been operating under apocalyptic religious delusions at the time of the killing. "He was also believing that he was Jesus and there was going to be an apocalypse," Pirelli said.

According to court records, Hertgen spent the evening of Feb. 22, 2025, with another brother, identified only as "D," who later told investigators Matthew had been "distressed, despondent, and experiencing terrifying visions" throughout the day. D dropped Matthew off at the apartment around 10 p.m. and briefly met Joseph there before leaving.

At 10:10 p.m., D texted Joseph warning him that Matthew was struggling with "his visions" and to reach out if he needed help. Less than an hour later, at 11:16 p.m., Hertgen called 911 to report a fire and a body inside the home — telling dispatchers his brother had been dead on the floor for 30 to 40 minutes. When asked how Joseph was injured, he said, "I'm not gonna go into any more details right now."

Officers arriving at the scene found Joseph Hertgen on the floor surrounded by a large pool of blood. He had suffered severe blunt force trauma to the back of his head, a large laceration to his chest and abdomen that exposed the chest cavity, and lacerations to his right eye — his right eyeball was not visible. A golf club was found on the floor near the body. On the dining table, police found a cup and utensils coated in what appeared to be blood.

In the living room, officers found the family's cat dead on an ottoman. The animal had been beaten, stabbed and set on fire.

Hertgen told police he had "gone into a fit of madness, maybe like forty minutes ago." He never denied his role in his brother's death.

His sentencing is scheduled for May 1 in Mercer County Superior Court.

Have a correction or news tip? Email sarah.salvadore@patch.com