Mr. Gilbert, 34, attended private boarding schools on the Upper East Side and in New England, graduated from Princeton with an economics degree and was expected to follow in his father’s footsteps to a lucrative Wall Street career, witnesses said.

His father tried unsuccessfully to bring him into the family business, Wainscott Capital Partners.

But Mr. Gilbert never flourished. Instead, he spent years living off his parents while he surfed, traveled the world and attended exclusive social clubs in Manhattan and the Hamptons.

He was diagnosed with several mental illnesses, including obsessive compulsive disorder, depression and schizophrenia, and his parents sought help from doctors who prescribed him medication that he refused to take, according to testimony.

His mother, Shelley Gilbert, recalled on the witness stand feeling delighted when her son, whom she referred to adoringly as Tommy, showed up unannounced at their Turtle Bay apartment on the afternoon of Jan. 4, 2015.

“He came in and told me he wanted to talk to Dad about business and so I was excited about that,” she said.

She testified that she had not seen her son in months, so she obliged when he asked her to go out to buy him a sandwich and a Coke, a brand of soda he knew his parents never stocked in the apartment.

On her way out, she went to the bedroom where her husband was watching a sports game from his bed.

She quietly put on a pair of sneakers and left the apartment without saying a word to her husband about her son’s arrival. She told the jury she wanted father and son to sort out their differences on their own.