The preppy Princeton, New Jersey man who allegedly killed his younger brother in the family’s upscale apartment also set the family cat on fire during the attack, a law enforcement source tells The Post.

A Princeton man has been charged with killing his brother and a pet cat in an apartment Saturday night, authorities announced Monday. Matthew Hertgen, 31, is charged with first-degree murder and related weapon possession charges in the death of Joseph Hertgen, 26, the Mercer County Prosecutor’s Office said in a statement.

A former college soccer player is accused of killing his younger brother and a cat, New Jersey prosecutors announced Monday, Feb. 24. The Mercer County Prosecutor’s Office said charges have been filed against Matthew Hertgen, 31, for first-degree murder and weapons offenses in connection with the death of his 26-year-old brother, Joseph Hertgen.