New York (CNN) Once part of Manhattan's social elite, a 30-year-old Princeton graduate lived off his parents, prosecutors say, spending his time surfing in the Hamptons -- until he killed his father over a cut to his weekly allowance.

"It wasn't until the dad started cutting off his money that the defendant decided to kill him," said Craig Ortner, an assistant district attorney with the Manhattan district attorney's office. "Literally hours before he was killed, (his father) further cut his allowance to $300."

Thomas Gilbert Jr., now 34, has declined to appear in person for much of his five-week trial on charges including murder and weapons possession, but he was present for closing arguments Wednesday, wearing a white button-down shirt and slacks, his gaze lowered.

Tommy, as he is known to family and friends, is being held at the Rikers Island prison. Defense attorney Arnold Levine says his client is not guilty by reason of insanity and has refused to cooperate with his own lawyers.

His father, Thomas Gilbert Sr., was a founding managing partner at Wainscott Capital, a lucrative hedge fund, prosecutors say.

Thomas Jr. is a former Princeton football player who graduated with a degree in economics. At the time of the shooting in January 2015, he was 30 and unemployed, receiving up to $1,000 a week from his parents.

Prosecutors and his own attorneys do not dispute that he killed his father. Jurors will instead be tasked with deciding whether he is guilty of the charges -- which also include counts for forgery -- or not guilty by reason of insanity. The murder charge carries a sentence of 25 years to life in prison.

'Nobody wanted to think it'

Gilbert spent his younger years at expensive, highly selective East Coast private schools. His attorney showed jurors snippets of his grades, SAT scores and teacher reviews from high school.

"Tommy's work -- indeed his whole manner -- this term has been the success story I still thrill to witness," wrote a teacher from his sophomore year.

But, Levine said, Gilbert "deteriorated" around the time he began to attend Princeton.

"Nobody wanted to look at him -- that background, those looks, that schooling -- and say he could be mentally ill," Levine said. "Nobody wanted to think it."

His former therapist, Susan Evans, testified that she saw him for several years before the shooting and that his preoccupation with paranoid thoughts was "interfering" with his ability to function.

"I don't believe he was working or ever worked during that time," Evans said. "Earlier on, he was preparing stock summary reports for a friend of his father's. I think that he didn't keep that up."

Evans recommended that Gilbert be screened for paranoid schizophrenia in the months before the incident. Other former doctors testified that they prescribed him antipsychotic medication. Evans testified that he did not take his prescribed medications for most of the time she saw him.

Emails between Gilbert and his parents that were shown in court depict a tumultuous relationship. He repeatedly asked for money, sometimes "for a charity thing," sometimes forwarding past-due bills from an exclusive athletic and social club for thousands of dollars to his mother.

In October 2013, Gilbert emailed his father, asking him for space.

"The more you respect my space, the more I will appreciate you!" he wrote.

Weeks later, though, he emailed his mother several times asking for hundreds of dollars for a garage, clothes and stock services. He bought a gun in the summer of 2014, prosecutors say, and a few months later, his father began discussing lowering his allowance.

A voicemail from Thomas Sr. describes his desire to foster financial independence in his son, but the defense noted that Thomas Sr. had thousands of dollars in credit card debt.

In October 2014, prosecutors say, Gilbert's computer showed searches for websites that discussed forging checks and offered blank check templates. Some of the charges he faces stem from credit card forging devices found at his home at the time of his arrest, prosecutors say.

In November 2014, prosecutors say, his computer showed searches for websites like "Hire-a-Killer.com" and "Find-a-Hitman.com."

Shortly after, Thomas Sr. emailed his son, telling him his weekly allowance would be reduced from $800 to $600. The day of his death, he changed his regular deposit into his son's account to $300, bank records showed.

The killing

On January 4, 2015, Shelley Gilbert found her husband on their bedroom floor in their Manhattan apartment with a gunshot wound to his head and a gun on his chest.

Minutes before, Gilbert had come in and cleared his mother out of the apartment, asking her to get him a sandwich and a drink, she testified.

Levine has argued that his client was clinically insane during the shooting. But prosecutors and their medical experts say he knew that what he was doing was wrong.

Prosecutors point to surveillance footage that shows Gilbert, wearing a hoodie and carrying a gym bag, entering his family's posh apartment building just after 3 p.m. and leaving less than 15 minutes later.

Forensic psychiatrist Jason Hershberger, who did not personally examine Gilbert because he declined to be seen, testified that he has read Gilbert's medical records and records related to the incident.

"I know that he shot his father," Hershberger testified. "I know that his father was found as if he had shot himself. The gun was in his hand as if he had committed suicide. Then Thomas left the scene. He turned off the location monitor [on his phone]. He was in his home in the dark when the police came to knock on his door. He pretended he wasn't there."

Hershberger testified that he does not believe there is evidence to support Gilbert's plea of not guilty by reason of insanity.

"In my opinion, anyone who attempts to conceal his behavior around a crime does so because they're aware of what they're doing is wrong," he testified.

In New York, a defendant must prove their inability to distinguish right from wrong in order to win an insanity plea defense.

Gilbert was deemed fit to stand trial after undergoing four mental evaluations, one of which was inconclusive.

His attorney argues that Gilbert's family tried to get help for his mental illness for over a decade.

"He faked being well for 12 years," Levine said. "He didn't fake being sick; he faked being well."

Shelley Gilbert's 911 call was played in Manhattan Supreme Court.

When an emergency dispatcher asked who shot her husband, she replied, "My son, who is nuts, but I didn't know he was this nuts."

CORRECTION: An earlier version of this article had the wrong first name for Thomas Gilbert Jr.'s attorney. He is Arnold Levine. A reference to credit card debt should have referred to the father, not the son.