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.

A former college soccer player accused of killing his younger brother and a cat apparently posted a violent poem on social media months before the alleged attack.