Mangione, 27, made the remarks as court officers escorted him out of the courtroom after a judge scheduled his state murder trial to begin on June 8, three months before jury selection in his federal case.
Judge Gregory Carro, matter-of-fact in his decision after a lengthy discussion with prosecutors and defence lawyers at the bench, said the state trial could be delayed until September 8 if an appeal delays the federal trial.
Mangione's lawyers objected to the June trial date, telling Carro that at that time, they will be consumed with preparing for the federal trial which involves allegations that Mangione stalked Thompson before killing him.
"Mr Mangione is being put in an untenable situation," defence lawyer Karen Friedman Agnifilo said.
"This is a tug-of-war between two different prosecution offices."
"The defence will not be ready on June 8," she added.
"Be ready," Carro replied.
Mangione has pleaded not guilty to state and federal charges, both of which carry the possibility of life in prison.
Last week, the judge in the federal case ruled that prosecutors cannot seek the death penalty.
Wearing a tan jail suit, Mangione sat quietly at the defence table until his outburst at the end of the hearing.
Jury selection in the federal case is set for September 8, followed by opening statements and testimony on October 13.
As the trial calendar began to take shape, Manhattan prosecutors last month urged Carro to set a July trial date in the state case.
In a letter, Assistant District Attorney Joel Seidemann asked Carro to begin the New York trial on July 1, arguing that the state's interests" would be unfairly prejudiced by an unnecessary delay" until after the federal trial.
"It appears the federal government has reneged on its agreement to let the state, which has done most of the work in this case, go first," Carro said on Friday.
Mangione is not due in court again in the state case until May, when Carro is expected to rule on a defence request to exclude certain evidence that prosecutors say connects Mangione to the killing.
Those items include a 9mm handgun that prosecutors say matches the one used to kill Thompson and a notebook in which they say he described his intent to "wack" a health insurance executive.
Last week, the judge in the federal case, Margaret Garnett, ruled that prosecutors can use those items at that trial.
In September, Carro threw out state terrorism charges but kept the rest of the case including an intentional murder charge.
When Mangione was arrested, federal prosecutors said they anticipated the state case would go to trial first.
Under New York law, the district attorney's office could be barred from trying Mangione on state murder charges if his federal trial happens first.
The state's double jeopardy protections kick in if a jury has been sworn in a prior prosecution, such as a federal case, or if that prosecution ends in a guilty plea.
Thompson, 50, was killed on December 4, 2024, as he walked to a midtown Manhattan hotel for UnitedHealth Group's annual investor conference.
Surveillance video showed a masked gunman shooting him from behind.
Police say "delay," "deny" and "depose" were written on the ammunition, mimicking a phrase used to describe how insurers avoid paying claims.
Mangione, a University of Pennsylvania graduate from a wealthy Maryland family, was arrested five days later at a McDonald's in Altoona, Pennsylvania, about 370km west of Manhattan.
