Prosecutors accuse Thomas Gilbert Jr. of killing his father over a decrease in allowance. (Courtesy of Manhattan District Attorney's Office)

It started with a cut to his allowance, prosecutors said.

Then-30-year-old Thomas Gilbert Jr. fatally shot his 70-year-old father in the head hours after his already tightened weekly money was decreased to $300, according to the Manhattan District Attorney’s Office.

Four years later, on Friday, a jury convicted the Princeton graduate on one count of murder in the second degree and two counts of criminal possession of a weapon, the district attorneys said. They rejected the defense’s argument Gilbert was insane at the time he killed his father and was therefore innocent.

As the trial stretched over several weeks, the prosecution painted a picture of a privileged son who attended a string of elite schools and spent his parents’ money on golf and country clubs without holding a job, the New York Post reported. Gilbert relied on as much as $1,000 per week from his parents, the district attorney said, according to CNN; emails displayed in court showed Gilbert asking multiple times for money and sending his mother overdue bills for thousands of dollars.

“The defendant rejected hard work, instead preferring an easy life handed to him on a silver platter,” Manhattan Assistant District Attorney Craig Ortner said at trial, according to the New York Post.

The defense, meanwhile, pointed to Gilbert’s documented history of mental illness stretching back to 2004 — the first time the defendant got treatment but not the first time trouble surfaced, defense attorney Arnold Levine told The Washington Post prior to the verdict. He did not immediately respond to a request for comment after the jury’s announcement.

Gilbert faces up to a life in prison for the murder. Sentencing is set for August. He was exonerated of an additional charge of criminal possession of forgery devices.

The elder Thomas, a managing partner at a New York hedge fund, raised the idea of decreasing his son’s allowance in 2014, a few months after Gilbert bought a gun, CNN reported the prosecution as saying. The district attorney also said the defendant made searches in the fall of 2014 for sites discussing check forgery — and, more ominously, for “Hire-a-Killer.com” and “Find-a-Hitman.com,” according to CNN.

On Jan. 4, 2015, the district attorney said, not long after the father decreased the allowance to $300, Gilbert killed his father in his family’s apartment and left him on the bedroom floor, the gun on the dead man’s chest, according to the complaint document.

His mother, Shelley Gilbert, testified that she discovered the elder Gilbert minutes after her son asked her to get him food, CNN reported. In her call to 911, heard in court, Shelley Gilbert said the killer was her son.

“He is nuts, but I didn’t know he was this nuts,” she told authorities on the call.

The question of Gilbert’s sanity is at the heart of the case, as the defense agrees the defendant shot his father.

Despite his record of academic achievement, Levine said at trial, Gilbert began to struggle mentally in college.

“Nobody wanted to look at him — that background, those looks, that schooling — and say he could be mentally ill,” Levine said, according to CNN. “Nobody wanted to think it.”

The defendant’s former therapist said at trial she had recommended he get tested for paranoid schizophrenia leading up to the killing and noted her client did not work while she saw him for several years. The therapist also said Gilbert repeatedly failed to take his medications.

The prosecution brought an expert of its own to counter the claim of insanity: forensic psychiatrist Jason Hershberger. After examining details of the case and Gilbert’s medical records, he testified, he did not believe the defendant was insane at the time of the alleged crime because he was lucid enough to try to evade authorities — for example, by placing the gun as if Thomas Sr. had shot himself, and by turning off location tracking on his phone after leaving his family’s apartment.

“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,” Hershberger said, according to CNN.

Levine, on the other hand, told The Post Hershberger’s testimony was “baseless” because he had not interviewed the defendant or others who knew him. Gilbert has refused to cooperate even with his own lawyers and the experts they have found, according to Levine, who said Shelley Gilbert hired him.

The prosecution’s “expert was able to reach an opinion anyway, even though the defense expert who tried to meet with [Thomas Jr.] six times couldn’t reach an opinion because the client wouldn’t speak with him,” Levine said.

Hershberger did not immediately respond to a request for comment. The Manhattan District Attorney’s Office declined to comment beyond confirming basic facts of the case.

Gilbert, who is now 34, is being held in Rikers Island prison. He did not appear for much of the trial, Levine said, although he was present for concluding arguments Wednesday.