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. “It was gruesome. It was way overboard, he even killed the cat, setting him on fire,” said a cop source at the scene. Police entered the Michelle Mews apartment complex — where units fetch upwards of $2 million — after Matthew Hertgen called 911 to report a fire and a dead body inside the family residence. Once inside, they found Joseph’s bloodied corpse along with the dead cat and quickly arrested Matthew, 31, a former Wesleyan soccer player, and charged him with murder. “The brutality of the homicide and in an Ivy League town shocked most detectives,” said a high-ranking NJ police source. Joseph, 26, was a superstar soccer player, garnering numerous accolades beginning at Toms River North High School and continuing at the University of Michigan, where he was a three-time Academic all-Big Ten before graduating in 2020. The brothers came from a wealthy family, who moved from a $1.1 million home in Toms River to ritzy Princeton. “It’s incredibly tragic. Matthew Hertgen came from what appeared to be a perfect, all-American family. No one could have predicted that something like this would happen. To have it end in such violence is shocking, and the loss of his brother Joseph is devastating,” another high-ranking police source said. Authorities have charged Matthew with first-degree murder, weapons offenses and animal cruelty. If convicted, he faces up to 30 years to life in prison. Prosecutors said Joseph’s official cause of death will be determined pending the results of an autopsy. Matthew posted a bizarre Facebook photo of himself with a $7.99 cat toy just one day before the rampage. In the un-captioned image, he sports a far-off star in his eyes and a shaggy beard.