His millionaire father cut his allowance by US$200 (S$270) and it caused Thomas Gilbert Jr, 30, to apparently fly into a rage.
So he went to his father's New York apartment on Sunday and asked his mother to get him a sandwich. While she was out, he shot his father dead with a single shot to the head, the New York Post reported.
"He was cutting the son's allowance. He had been giving him US$2,400 a month for rent and US$600 for spending money, and he was cutting that to US$400 a month," a source said about Thomas Gilbert Sr, founder of the Wainscott Capital hedge fund.
"They had argued about it before."
Gilbert Sr, 70, was a respected figure in Wall Street, the report said.
Sources said they found the box the gun came in after Gilbert Jr surrendered at his apartment on Sunday night. Police also found two magazine clips and loose rounds scattered around his Chelsea pad.
It is still unclear whether he bought the gun legally. Authorities have charged the son.
The body of Gilbert Sr was discovered in a bedroom of his apartment in Manhattan's East Side, Bloomberg reported.
NYPD Chief of Detectives Robert Boyce told a press conference that the two men argued about money before the son allegedly shot his father in the head.
When the elder Gilbert's wife returned, she discovered her husband had been shot and that a gun was lying on his chest and covered with his hand, Detective Boyce said.
"The gun laying where it was, it didn't seem it was a self-inflicted wound," Detective Boyce said, who also described the apartment as a "staged crime scene", Reuters reported.
The Wall Street Journal reported that the victim's wife called emergency responders after finding the body.
"I heard a sound right above me," Mr Christopher Kelly, who lives below the Gilberts, told the New York Post. "I thought somebody dropped something."
Gilbert Sr was a graduate of Princeton University and Harvard Business School, according to his biography on the Wainscott Capital Partners site.
He founded Wainscott in 2011, after leaving Syzygy Therapeutics, a private-equity biotech asset acquisition fund.
"He was a pillar on Wall Street - somebody everyone looked up to," another neighbour, Hector Torres, told the New York Post. "Very nice gentleman… they seem like a normal family. Every time I saw him in the elevator, he would say, 'Hi'. He would ask how I was."
Gilbert Jr, a graduate of Deerfield Academy and Princeton University, according to his Facebook page, had made preparations to follow in his father's money-management footsteps.
This article was first published on January 7, 2015.
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