The Mercer County Prosecutor’s Office and the Middlesex Regional Medical Examiner’s Office did not immediately respond to PEOPLE’s requests for comment on Sunday, May 24.

Matthew’s death comes after a judge ruled in March that he was not legally responsible for his 26-year-old brother Joseph Hertgen’s death on Feb. 22, 2025, due to severe mental illness, NJ.com reported.

The judge accepted that Matthew was legally insane and scheduled a May hearing that would have determined his future custody, the outlet reported.

The New Jersey native had a documented history of escalating psychotic behavior, Dr. Gianni Pirelli, a forensic psychologist, testified in court.

Dr. Pirelli said that Matthew experienced “prophetic and divine visions” of a religious and apocalyptic nature, at times believing that he was God or the anti-Christ, or that a “sacrificial murder” was necessary to save the world, according to the New York Post.

“Anytime he closes his eyes, he’s seeing tremendous visions,” Pirelli said of Matthew while testifying, per the outlet.

Joseph Hertgen in 2016.

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Matthew faced a first-degree murder charge and weapons offenses in connection with his brother’s death, which occurred at a Princeton, N.J. apartment, as well as animal cruelty for killing a cat who lived in the residence.

He was accused of using a knife and golf club to attack Joseph before ripping out his eye and setting fire to the animal.

He also ate his brother’s eyeball, per the New York Post. A plate and utensils smeared with blood, and a small cup filled with what appeared to be blood, were found inside the home, per a probable cause affidavit previously reviewed by PEOPLE.

During the course of their investigation, officials spoke with Matthew and Joseph’s older brother, David Hertgen, who told them Matthew had been experiencing severe mental illness for five years and that he’d been “extremely distressed, despondent and experiencing terrifying visions” on the day of Joseph’s death, per the affidavit.

On the night of the murder, the older brother texted Joseph, telling him that Matthew was having “visions” and to let him know if he needed help. Joseph responded, “Will do,” according to the affidavit.

In his obituary, Matthew’s loved ones underscored his mental health struggles and remembered who he was during his upbringing.

The family also encouraged donations for NAMI Mercer, a nonprofit that works with residents of Mercer County, N.J., that are affected by mental illness.

Matthew Hertgen in court on March 6.

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“Growing up, Matthew is remembered as a caring and loving person,” the obit reads. “He was blessed with a large and extended family who he enjoyed spending time with. Matthew attended Wesleyan University in Connecticut where he played soccer and graduated with a Bachelor of Arts degree in microbiology and biochemistry. He went on to work as VP in a senior finance capacity for a telecom company for many years.”

“During his later years, Matthew struggled with severe and profound mental health issues; yet he expressed sorrow, remorse, and repentance in many ways,” the obit continues. “He departed with the love, friendship and forgiveness of his family and the eternal hope of salvation. May he rest in peace.”