Princeton Man Ruled Legally Insane in Brother's Murder

Matthew Hertgen will likely be committed to psychiatric facility after judge's ruling.

Mar. 21, 2026 at 2:49am

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In a bench trial, a judge ruled that Matthew Hertgen, 32, was legally insane when he killed his brother Joseph, 27, at their family's apartment in Princeton's Michelle Mews in February 2025. Hertgen's defense and the prosecution agreed that he was having visions of a spirit at the time of the murder. The judge said Hertgen was "laboring under such a defect of reason from disease of the mind" that the act was not morally wrong in his "delusional state."

Why it matters

This case highlights the complex legal and mental health issues surrounding crimes committed by individuals with severe mental illness. The insanity defense is rarely successful, but the judge's ruling suggests Hertgen's delusions and psychotic state at the time of the killing made him not criminally responsible.

The details

On February 22, 2025, Hertgen killed his brother Joseph, 27, at their family's apartment in Princeton's Michelle Mews. Forensic psychologist Gianni Pirelli testified that Hertgen was having visions of a spirit at the time of the murder. In the bench trial, the prosecutor agreed that Hertgen had established the insanity defense, and the judge ruled he was not criminally responsible due to his "defect of reason from disease of the mind."

  • On February 22, 2025, Hertgen killed his brother Joseph.
  • On March 18, 2026, the judge ruled Hertgen was legally insane at the time of the murder.
  • Sentencing is scheduled for Friday, May 1, 2026.

The players

Matthew Hertgen

A 32-year-old Princeton resident who was ruled legally insane in the murder of his brother Joseph.

Joseph Hertgen

Matthew Hertgen's 27-year-old brother, who was killed by Matthew in their family's apartment.

Judge Robert Lytle

The judge who presided over the bench trial and ruled that Matthew Hertgen was not criminally responsible for his brother's murder.

Gianni Pirelli

A forensic psychologist who testified that Matthew Hertgen was having visions of a spirit at the time of the murder.

Tim McCann

The prosecutor who agreed that Hertgen had established the insanity defense.

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What they’re saying

“Anytime he closes his eyes, he sees tremendous visions.”

— Gianni Pirelli, Forensic Psychologist

“It's the state's position the defendant has established the insanity defense by preponderance. The state concedes, and really can't contest that at this time.”

— Tim McCann, Prosecutor

“In his delusional state, the act was not wrong in the ordinary moral sense. It was required. The court finds the defendant is not criminally responsible for his actions because at the time of the relevant conduct, he was laboring under such a defect of reason from disease of the mind.”

— Judge Robert Lytle

What’s next

Sentencing for Matthew Hertgen is scheduled for Friday, May 1, 2026, where he will likely be committed to a psychiatric facility.

The takeaway

This case highlights the complex legal and mental health issues surrounding crimes committed by individuals with severe mental illness. The successful insanity defense in this case suggests the judge recognized Hertgen's delusions and psychotic state made him not criminally responsible for his brother's murder.