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 star has been charged with killing his younger brother and a pet cat inside a New Jersey apartment complex near the Princeton University campus, according to officials.

The preppy Princeton, New Jersey man who allegedly murdered his younger brother and butchered the family feline inside their luxury apartment posed with an $8 cat toy the day before the gruesome killings.

Matthew Hertgen, of Princeton, was charged with the first-degree murder of 26-year-old Joseph Hertgen, as well as animal cruelty and weapons offenses related to possession of a knife and a golf club, police said.

The Princeton, New Jersey, man accused of killing his former college athlete brother and a cat posted eerie messages to social media months ago.

Matthew Hertgen allegedly killed his younger brother, Joseph, and is now facing various charges including first-degree murder and one count of animal cruelty.

A former college soccer star is being charged with killing his younger brother and the family cat in an apartment near Princeton University. Matthew Hertgen, 31, is in police custody in Mercer County,

The New Jersey man who allegedly beat and stabbed his younger brother to death in a ritzy Princeton apartment posted a creepy poem to social media that included a