The court's liberals and conservatives were on opposite sides about the reach of two of the court's precedents: a 2012 ruling that said states may not mandate life without parole for juvenile killers, and a subsequent decision that said the ruling was retroactive.
Malvo, who along with his adult mentor John Allen Muhammad killed 10 people in the Washington region in random sniper attacks, believes his Virginia sentences qualify. But even his lawyers say a victory for him would likely have more value for others. Malvo also received six life sentences for Maryland murders, and he could be prosecuted in other states.
Justice Samuel Alito was one of the biggest skeptics.
"So, if he can demonstrate, as a result of good behavior in prison, for example, that he has been rehabilitated, then he must be released?" asked Alito.
"No. No, absolutely not," said Washington lawyer Danielle Spinelli. She said that even with new hearings, "there are occasions when juvenile offenders are resentenced to life without parole. Even if he were given parole eligibility, that would not mean that he would be released."
Beyond Malvo, the case could signal an expansion - or a retreat - from the court's rulings that say even the most violent juvenile offenders must be treated differently. But those rulings came in cases where Justice Anthony Kennedy joined the court's liberals.
Kennedy retired last year and has been replaced by Justice Brett Kavanaugh, and it appeared from questioning in the hour-long oral argument that Kavanaugh will play a decisive role in the outcome.
Virginia Solicitor General Toby Heytens argued that Malvo was not entitled to a new hearing. The Supreme Court's precedents, he said, were founded on the notion that mandatory life without parole for juvenile offenders violated the Eighth Amendment's prohibition on cruel and unusual punishment.
Virginia's Supreme Court has found that the commonwealth's law is not mandatory.
But the court's liberals found that decision questionable. The jury that convicted Malvo for the murder of Linda Franklin outside a Home Depot store in Fairfax County, Virginia, was deciding between recommending life without parole and the death penalty, which in 2004 had not been ruled out for juveniles.
"The jury had only two choices: death or life without parole," said Justice Ruth Bader Ginsburg. "And nobody seemed to have appreciated at the time of Malvo's convictions that there was any discretion."
She asked whether any Virginia judge ever reduced the life sentence for a juvenile. Heytens said he did not know of any.
Justice Sonia Sotomayor also pressed about a judge's discretion. "That really is the nub of this case," she said.
Justice Elena Kagan wrote the majority opinion in Miller v. Alabama that said mandatory life sentences without parole were prohibited for juvenile offenders. She told Heytens that it required more than a determination of whether the sentence was mandatory.
"It's a 30-page opinion, and it can be summarized in two words, which is that 'youth matters,' and that you have to consider youth in making these sorts of sentencing determinations," Kagan said. "Of course, it talks a lot about mandatory schemes because a mandatory scheme was in front of it, but the entire reasoning was about how much youth matters and how a judge or a jury, whoever the sentencer is, has to take that youth into account."
Kavanaugh said one way to look at the Supreme Court's precedents is that "you can't impose life without parole on someone who's merely immature as opposed to incorrigible." And if that's the rule, he asked, is it enough that a state has a discretionary sentencing scheme, or must the sentencer indicate in some way that question has been considered?
Justice Department lawyer Eric Feigin said the U.S. Court of Appeals for the 4th Circuit, which ruled for Malvo, was wrong when it said a discretionary scheme was not enough. It is, Feigin said, although the department believed the case should be remanded for another look at whether that accurately characterizes Virginia's law.
Spinelli reminded the court that Malvo's sentencing took place before any of the court's relevant rulings on juvenile offenders. She said that Virginia's laws were not discretionary, but that "the theoretical opportunity to consider youth, when it wasn't actually considered, simply can't be squared with the language of" the court's more recent precedents.
Kavanaugh pointed out that the precedents don't require a finding of fact, only consideration. And it is presumed that judges consider the factors that are required.
Spinelli said: "But this is not a case where the judge was required to consider anything. And, in fact, she did not consider imposing any lesser sentence than life without parole."
Alito and Justice Neil Gorsuch pushed Spinelli on whether the court's precedents were as broad as she read them; both seemed to say it was enough to find that a sentencing scheme was discretionary.
Spinelli resisted. "The question is whether the juvenile committed the crimes based on transient immaturity or permanent incorrigibility," she said. "And what we are asking for is a hearing in Virginia court where the Virginia sentencer will make that determination."
Muhammad met Malvo when the boy was an abandoned 14-year-old in Antigua. He brought him to this country, trained him as a sniper and enrolled him in a bizarre terror scheme.
They shot at least 22 people across the country, killing 14 of them, including the 10 homicides in the Washington area. Muhammad was executed in Virginia in 2009.
Since the Supreme Court's rulings, advocacy groups say about 1,950 inmates have received new hearings or relief from new laws. Twenty-two states and the District of Columbia have prohibited life sentences without parole for juveniles.
Malvo, now 34, is one of 12 people sentenced as juveniles and serving life in Virginia.
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