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Years before Thomas Gilbert Jr. was accused of shooting his father in the head, he told a psychiatrist that he dreamed about kicking his father and screaming, “Stop it! Stop it!”

Even though his father had supported his lavish lifestyle — rent, overseas trips and a $1,000 weekly allowance — Mr. Gilbert continued to have angry delusions about his father, according to psychiatrists who treated him in the past.

Their assessments came during the fourth week of testimony in Mr. Gilbert’s murder trial. Prosecutors contend the now 34-year-old Princeton University graduate showed up unannounced at his parents’ Turtle Bay apartment on Jan. 4, 2015, and minutes later fired a bullet from a handgun into his father’s temple.

He was livid that his father, Thomas Gilbert Sr., had cut two-thirds of his allowance, prosecutors said. The body was posed afterward to make the death look like a suicide.