Kevin Reddington, a veteran trial lawyer, kicked off the defense of his client earlier this summer with an audacious assertion, given that his client is Lindsay Clancy, the Massachusetts nurse who had admitted to strangling her three young children. “This,” Mr. Reddington told 12 jurors in his opening remarks, “is not a righteous prosecution.” Eleven of those jurors voted not to convict Ms. Clancy, 36, who claimed she was suffering from postpartum psychosis at the time of the killings. With one holdout juror, the judge declared a mistrial, leaving a question mark hovering over a murder case that set off a global conversation about the mental health burdens of motherhood, and long-held questions about the essential fairness of the insanity defense. A retrial is possible.

But for now, the mistrial must be considered a victory for Mr. Reddington, one that allows his client to return, for the time being at least, to a state-run psychiatric hospital rather than a prison cell. But if Mr. Reddington is to be believed, it will not register as a win for Ms. Clancy. During the trial, he described her as a mother well aware of what she had done to her children and “living her own hell.” Such statements by Mr. Reddington were part of a defense strategy that acknowledged Ms. Clancy’s dark actions but emphasized her mental health problems and the missteps of the medical establishment as she sought help.

In the wake of the six-week trial, a number of legal experts said that Mr. Reddington, 75, had masterfully pulled off an astonishingly difficult courtroom assignment amid the most macabre of circumstances. The jurors were forced to consider the horrors, not only of the killing, but also of Ms. Clancy’s attempted suicide, in detail so gruesome that Mr. Reddington apologized to them. “God bless you,” he said. “You people are going to have to listen to some of the most horrific things, look at some of the most horrific things.” Trying to convince jurors to consider an insanity defense is not easy in such circumstances. “There’s something in human nature that makes people want to assign blame,” said Shira M. Diner, who teaches at Boston University Law School.

There is a desire to mete out a punishment that “can make people feel, not better, but feel more at peace with the horrors that they learned about” she said. Even so, Mr. Reddington had an important factor that worked in his favor. Under Massachusetts law, it is up to prosecutors to prove beyond a reasonable doubt that a defendant like Ms. Clancy, who claims to be suffering from mental illness, was “criminally responsible” for her conduct at the time of killing — meaning that the defendant lacked “the substantial capacity either to appreciate the criminality or wrongfulness of his conduct or to conform his conduct to the requirements of the law.” R.

Michael Cassidy, a professor at Boston College Law School, said that Massachusetts “is one of a handful of states where, once the defense of ‘lack of criminal responsibility’ is raised, the burden is on the government to disprove it beyond a reasonable doubt. That’s not true in many states, and it’s not true in federal court. The defendant must prove lack of criminal responsibility by a preponderance of the evidence in most states, and in federal court, they have to prove it by clear and convincing evidence.” Mr. Reddington sought to instill a sense of doubt in the jurors from the outset. “The government says that this is a situation that’s not a referendum on women’s having babies and having postpartum psychosis, postpartum depression, but it sure as hell is,” he told the jury.

The strong language was a good introduction to jurors of Mr. Reddington’s style. He wore a dark suit on the first day of the trial, a likely acknowledgment of the gravity of the situation. But his language could be down-to-earth and occasionally salty — unsurprising for a man who has tried high-profile cases for decades but rides a Harley in his spare time.