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.

Matthew Hertgen’s mugshot, released Tuesday, shows the hairy-faced murder suspect with a thousand-yard stare painted on his face.

New Jersey authorities have arrested Princeton man Matthew Hertgen, who is accused of killing his brother, Joseph Hertgen, and a cat, allegedly with a knife and golf club.