"He might have had a change of heart and said, 'Wait a second, I'm going into state court. I'm admitting that I did this. How is this going to play when I'm facing even more serious charges in federal court in January of next year?'" Rice told Fox News Digital Friday.
The daring move would have required Mangione to concede that he shot Thompson, a 50-year-old father of two, at trial. If he were later convicted under New York's extreme emotional disturbance law, the murder charge would be reduced to manslaughter, and the potential sentence would be reduced from 25 years to life in prison to five to 25 years.
Speaking with Fox News Digital before Mangione's reversal, high-profile criminal defense attorney James Leonard said it would be "a very risky trial strategy for the defense.
"They are basically telling the jury that Mangione committed the murder, but here is why he did it and, because of this, you should nullify his guilt," he said. "If the jury accepts that, it would be an epic win for the defense team. If the jury rejects that, it [would] likely mean that Mangione will spend the rest of his life behind bars."
Rice said concerns about the federal prosecution likely factored into the decision.
"I have no doubt that this weighed on Mangione's mind," he added.
The former Ivy Leaguer is also facing a separate federal trial expected to kick off early next year, and it's unclear how such a defense in the state case would impact that one. It carries stiffer potential sentences, and there is no federal equivalent to New York's emotional disturbance law.
A spokesperson for Mangione's team did not immediately respond to a request for comment.
"We knew that this was gonna be one of these cases that's gonna have a lot of twists and turns, and it is certainly living up to that," Rice said.
Legal observers previously told Fox News Digital that the psych defense would have required Mangione's attorneys to convince jurors that he experienced a profound loss of self-control stemming from an intense emotional disturbance at the time Thompson was gunned down outside a Manhattan business conference.
Prosecutors, meanwhile, have alleged that Mangione meticulously planned the killing for months, documenting his thoughts in journals and traveling across the country before shooting Thompson in the back outside a business event in New York City, where neither of them lived.
"I have no doubt that this is not the last time we're going to see something like this where either the defense says something or even the prosecution may say something and then it completely gets changed and flipped on its head moving forward," Rice said.