The strategy poses steep legal hurdles but could lead to a jury convicting Mangione of the lesser crime of manslaughter, which carries significantly lighter sentences.
Mangione, who appeared in court in a dark suit and white shirt, is accused of fatally shooting UnitedHealthcare CEO Brian Thompson outside a hotel in Midtown in December 2024.
The brazen killing was widely condemned by US officials but became emblematic of the public's frustration with rising healthcare costs and health insurance industry practices.
Mangione pleaded not guilty in December 2024 to state murder, weapons and forgery charges brought by Manhattan District Attorney Alvin Bragg.
His trial is set for September before Justice Gregory Carro in Manhattan.
Under New York law, murder defendants can seek to convince a jury that their actions can be explained by an "extreme emotional disturbance" that reduces their criminal culpability.
Carro will ultimately decide whether there is enough evidence for the murder charge to be reduced.
Thompson led UnitedHealth Group's insurance unit before he was shot dead in the early morning outside a hotel where the company was holding an investor conference.
Graphic footage of the killing and a five-day manhunt for a suspect made the case a media fixture and social media sensation.
Mangione was eventually arrested in Pennsylvania.
Mangione separately pleaded not guilty in April 2025 to murder, weapons and stalking charges brought by Manhattan federal prosecutors.
US District Judge Margaret Garnett, who is overseeing that case, threw out the murder and weapons charges over legal technicalities in a surprise ruling in January.
That decision eliminated the possibility that Mangione would face the death penalty, although he still faces a possible sentence of life without parole if convicted of stalking.
Jury selection in that case is set to begin in September, and opening statements in the trial are scheduled for November.
