A 30-year-old man accused of killing his millionaire Manhattan father is a suspected arsonist and spoiled brat who may have murdered Daddy because the old man cut his allowance by a measly $200, sources said Monday.

Thomas Gilbert Jr. was already hiding a dark past — including his status as the current prime suspect in a Hamptons arson targeting a wealthy rival — before allegedly killing his respected Wall Street father, Thomas Gilbert Sr., 70, sources told The Post.

The pampered Princeton grad went on his alleged murderous rampage amid a months-long feud with his father over what he still thought was his rightful due — despite his parents giving him the best education money could buy and even subsidizing his surfer-dude, gym-rat lifestyle when he got older, sources said.

Gilbert Jr. regularly griped that “his dad was hypercritical of him — he couldn’t do anything right,’’ his ex-girlfriend Anna Rothschild told The Post Monday in an exclusive interview.

“He talked a lot about his dad and how mean he was to him and how nothing was good enough.”

He hadn’t seen his parents since August and finally had enough when Daddy cut his generous allowance, law enforcement sources said.

Gilbert Sr. “had been giving him $2,400 a month for rent and $600 for spending money, and he was cutting [the spending money] to $400,” a law enforcement source said. “They had argued about it before.”

Intent on reining in his son, Gilbert Sr., the Harvard-educated founder of Wainscott Capital, even planned to whittle down the $400 by another $100, a second source said.

At first, Gilbert Jr.’s mom, Shelley Gilbert, 67, was happy to see their son when he showed up on Sunday at the door of their $6,000-a-month apartment at 20 Beekman Place near the United Nations at about 3:30 p.m., sources said. She thought he had finally come to make peace with his father, they said.

The doting mom offered to make Gilbert Jr. something to eat, but he brusquely told her, “I want to talk to Dad,” and asked her to go buy him a sandwich and soda instead, sources said.

But it was a ruse, they added.

“He asked her to get him a sandwich so she wouldn’t be there,” a source said.

When the mom returned, she found her husband’s body in their bedroom — shot once in the head — and their son gone, police said.

“My husband’s been shot by my son!” sources said the distraught woman told a 911 operator.

Cops believe Gilbert Jr. tried to make the murder look like a suicide, placing his .40-caliber Glock semiautomatic on his father’s chest, with the victim’s left hand covering it, before fleeing, sources said.

Cops found the gun’s case — along with two clips and numerous loose rounds — in Gilbert Jr.’s apartment at 350 W. 18th St. after busting him there at about 11 p.m. Sunday, sources said.

Police were trying to determine if he bought the weapon legally.

After the killing, Gilbert watched TV in his pad, law enforcement sources said.

The son may have been planning a getaway. An open suitcase filled with clothes and lying on the hallway floor of his first-floor apartment could be seen through the home’s peephole Monday.

At his arraignment Monday night — at which he was ordered held without bail on charges of second-degree murder and weapons possession — prosecutors said Gilbert was found with a skimming device and 21 blank credit cards.

The tall, handsome Gilbert Jr. — a graduate of the Buckley School on the Upper East Side, Deerfield Academy in western Massachusetts and Princeton — was a fixture on Manhattan’s black-tie society circuit.

But Rothschild, who dated Gilbert Jr. for four months in early 2014, described him as a troubled man who had few friends and no great job prospects.

Gilbert Jr. was trying to start his own hedge fund, but “his dad wouldn’t help him and told him he was stupid,” she said.

“He’s very much a loner. His phone never rang. No one texted him or called him. The one time it rang in four months while we were together, it was his mother.”

Another source said the son suffered from severe obsessive-compulsive disorder and had been off his medication at the time of the slaying.

Gilbert Jr. spent most of his time hanging out in the Hamptons, Rothschild said.

“He surfed and went to the gym and went to yoga, he was very fit,’’ she said.

Rothschild said that she broke off their relationship in May and that she last saw Gilbert Jr. around Christmas.

“We had drinks. He was totally normal. I asked him about the hedge fund. He said it was moving along now. He was in good spirits,’’ she said.

“How could a guy be that gorgeous, that wealthy, that fit, and kill his dad? This is the last thing in a million years that I thought he could do.’’

But even Gilbert’s brokenhearted sister, Clare, 24, told authorities her father could be very controlling when it came to money, sources said.

Gilbert Sr. ran a hedge fund with about $10 million in assets and owned a multimillion-dollar home in the Georgica Association, a gated community in East Hampton, records show.

The dad may have had reason to keep a careful eye on his dough, sources said.

He launched Wainscott Capital Partners in July 2011 with an estimated $2 million of his own money but struggled to get outside investors, sources said.

By January 2012, he had raised only $575,000 from one outside investor, a regulatory filing shows.

Gilbert named his fund after the town in the Hamptons where he owned a home — which he borrowed $4 million against last year.

Meanwhile, Gilbert Jr.’s behavior grew worse over the summer and into the fall, sources said.

Gilbert Jr. was questioned in a blaze that destroyed a 17th-century house on Sagg Main Street in Sagaponack on Sept. 15 and is still the main suspect, although he has not been charged, sources said.

The home was owned by millionaire ex-Lazard financier Peter Smith Sr., whose son, Peter Smith Jr., had had a falling-out with Gilbert Jr. over a girl, a source said. Smith Jr. already had a restraining order against Gilbert Jr., a court source said.

Gilbert was suspected of killing Smith’s dog before the fire, a source added.

The two families knew each other from the exclusive Maidstone Club overlooking the Atlantic in East Hampton, whose members have included many of New York’s most prominent families, such as Jacqueline Kennedy’s father, John “Black Jack” Bouvier III.

But Gilbert Jr. had been blackballed from it after a beef.

He also had gotten into trouble at Stephen Talkhouse and other local clubs, sources said.

Gilbert Jr. started seeing a shrink in December, but it was unclear if he was still under a psychiatrist’s care in the days before the shooting, sources said.

Neighbors described his father as an affable and hearty man.

“I knew him well,’’ said Peter Gazarian, 75. “He always smiled and bent down to pet my little dog, Nina, always had a kind comment from him.

“He was a tall, strong-looking man, always elegantly dressed.”

Additional reporting by Michelle Celarier, Jennifer Gould Keil, Frank Rosario and Bob Fredericks