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

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.

Chilling social media posts from Matthew Hertgen -- the man accused of killing his brother Joseph and a cat inside a New Jersey apartment -- have come to light.