But Thomas Gilbert's lawyer, Arnold Levine, has argued that Gilbert was too mentally ill to be held responsible for the murder. He summoned doctors to the stand in an effort to convince a Manhattan Supreme Court jury his client suffered from a long list of mental illnesses, including paranoia.
In New York, defendants who present an insanity defence must prove that mental illness prevented them from understanding they committed a crime or knew that their actions were morally wrong.
If history serves as a guide, Gilbert's lawyers face a tough road. Few defendants who use the defence win at trial, even when they have long-documented histories of mental disorders.
Prosecutors typically argue that the insanity claim is a ruse invented after a crime and seize on evidence that a killing was planned or the killer tried to cover it up to argue the defendant knew it was wrong.
Dr. Michael Sacks, a psychiatrist at Weill Cornell Medicine in Manhattan who treated Gilbert from 2008 to 2014, testified that his patient's delusions were so relentless he could not escape them even as he slept.
"He had a dream, a very unpleasant dream, that he was kicking his father and shouted, Stop it! Stop it!" Sacks said. "He felt his father was sadistic."
Sacks recalled jotting down that Gilbert thought his father was "messing with him" when he threatened to cut part of his allowance.
He eventually diagnosed him with severe compulsive disorder, depressive disorder, paranoid disorder, psychosis and other mental illnesses, he said.
The defence's psychiatrists told the jury that Gilbert's mental illnesses made it difficult for him to find employment after graduating with an economics degree from Princeton in 2009.
Unlike his father, who also graduated from Princeton and founded the hedge fund Wainscott Capital Partners, Gilbert accomplished little outside surfing and other hobbies.
But Craig Ortner, the lead prosecutor, has argued that while the younger Gilbert visited psychiatrists "like many New Yorkers," he was still a "self-absorbed" and "vengeful son" who planned to kill his father.
Ortner relied on Dr. Jason Hershberger, a psychiatrist, who said Gilbert's financial motive suggested he knew he was committing murder.
"In this case there is a reason, a real-world motive. If you look at the banking records and the relationship between Gilbert and his parents, they were cutting him off financially," Hershberger said. "It removes the support of insanity defence."
Even though Gilbert refused to speak with Hershberger, the doctor studied his health records and deduced that obsessive compulsive disorder — mainly the fear of being contaminated by objects and even people — took center stage throughout his life.
"He didn't have any evidence of hallucination in his record," he said. "Mental illness, even serious mental illness, is not enough to create that substantial incapacity."
Dr. Theodore Shapiro, a Weill Cornell Medicine psychiatrist, said it took him a handful of sessions to understand a paternal obsession was consuming Gilbert. Talking or being near his father caused him severe anxiety, he told Shapiro.
"His father was at the center of his delusions," the doctor said.
During cross-examination, Ortner pointed out that Gilbert was able control his hatred for his father when he needed money.
"He overcame his paranoia and asked his father to guarantee his apartment." Ortner said. "Did you know that?"
Shapiro paused, and then admitted, "Yes."
A pale and clean-shaven Gilbert sat quietly as he listened to part of his former doctors' testimonies. His behavior was a departure from his earlier courtroom flare-ups, which included spouting random objections and illogical legal phrases. The justice overseeing the trial, Melissa Jackson, has allowed him to come and go at will.
The mental health doctors brought in by the defence could not explain what event, if any, precipitated Gilbert's feelings about his father.
By all accounts, a young Gilbert thrived in his family's wealthy social circles. He attended the Buckley School in New York City's Upper East Side and Deerfield Academy in New England.
As he reached his teens, Gilbert bonded with his father during camping trips, his mother, Shelly Gilbert, testified early in the trial. But he also began interpreting mundane father and son interactions, like kicking a ball back and forth, as hostile, according to his doctors' testimony.
Those doctors testified this week that they had no choice but to drop the younger Gilbert as a patient in time because he refused to take medication.
"He didn't like how it made him feel," said Dr. Jason Kim, a Weill Cornell Medical psychiatrist who also saw Gilbert in 2014.
Not everyone was certain Gilbert's erratic behavior equated to mental illness. Peter Smith, who shared an apartment with Gilbert in 2012, said they parted ways after Gilbert punched him in the face unprovoked in October 2013, leaving him with "a bit of a busted nose."
"I could not tell you what exactly started it," Smith, a private lawyer, testified. "Tommy became very aggressive with me."
Several months later, police in the Hamptons described Gilbert as a "person of interest" in a fire that left Smith's family's 350-year-old home nearly in ashes. Southampton police officers arrested him on charges that he violated Smith's order of protection, but he was never charged with arson.
Asked by Levine if he thought those were the actions of a deranged man, Smith shrugged.
"He did a lot of things that were pretty normal," Smith said. "Did he do a lot of things that were disconnected from reality? I don't know."
Written by: Edgar Sandoval
Photographs by: Jefferson Siegel
© 2019 THE NEW YORK TIMES
