EVERETT — A man who as a teenager two decades ago snuffed out the lives of an Everett mother and her 12-year-old daughter learned Wednesday that he likely won’t see freedom until he’s in his 50s, at the earliest.

Brandon Backstrom must serve a minimum 42 years in prison for the killings he committed at 17, Snohomish County Superior Court Judge Ellen Fair said. Her decision brought clarity to the case after a U.S. Supreme Court ruling undermined Backstrom’s earlier sentence of life in prison without parole.

Backstrom, now 36, appears to have matured and improved himself during the 20 years he’s already spent behind bars, Fair said. But that doesn’t erase what he did.

“The underlying facts of these murders are particularly horrifying,” the judge said.

Backstrom was convicted of aggravated first-degree murder in the November 1997 slayings of Marnie Walls, 30, and her daughter, Korree Olin, of Stanwood. Each received more than 50 knife wounds. Walls, who was near death from long-term health problems, also was bludgeoned with a large pair of wire cutters.

The pair were killed to leave no witnesses to a robbery that netted some jewelry and cigarettes. Backstrom was tried as an adult because of the seriousness of the crimes. Upon conviction, a life sentence was the only punishment possible under the law.

Then, in 2012, the U.S. Supreme Court ruled that such mandatory life sentences for juvenile killers violate the Eighth Amendment’s prohibition of cruel and unusual punishment.

Washington’s law was amended in 2014 to reflect the high court’s ruling. Those who commit aggravated murder between the ages of 16 and 18 now face a minimum of 25 years behind bars. They still can be sentenced to life without parole, but only after a judge considers the individual circumstances of the case and the defendant’s life.

Fair on Wednesday made clear her hope is the sentence will stand this time. She took pains to list all of the information she’d considered, and the people she’d listened to, before reaching a decision on Backstrom’s punishment.

Among those who spoke were relatives of the victims.

Marnie Walls was struggling with illness and had recently been told that she likely only had a few months left, Fair was told. Her daughter was visiting that weekend so the young mother would have an opportunity to share the sad news.

The slayings were discovered by family, including Walls’ son, then just 10.

Casey Walls now has a family of his own. He told Fair he’s never fully shaken the shock or fear that Backstrom could harm other loved ones.

“Keeping him in will keep me at peace,” he said.

Backstrom was represented in the hearing by two public defenders, Paul Thompson and Fred Moll.

Thompson told Fair he was in an unusual position: a defense attorney compelled by circumstances to advocate punishment for a client who agrees that is appropriate.

The murders that led to Backstrom’s conviction can’t be minimized, the attorney said. At the same time, his client has changed over the years, and for the better.

“He’s simply not the kid who committed these most heinous acts,” Thompson said.

He urged Fair to sentence Backstrom to a minimum of roughly 30 years, the same punishment handed down to Backstrom’s co-defendant in the case. That man, who was 22 at the time of the murders, provided testimony that helped send Backstrom to prison.

Deputy prosecutor Matt Hunter told the judge how, in preparation for the hearing, he took advantage of a defense offer to meet with Backstrom at Clallam Bay Corrections Center. That’s the prison where the defendant has spent most of the past two decades, by all reports a model inmate.

Backstrom was likeable, polite and appeared to show genuine remorse, Hunter said. If set free, he doubted the man would reoffend.

But that’s only one consideration, the prosecutor said. He argued the defendant’s acts still demand punishment, particularly when he’s yet to take full responsibility for his victims’ fatal injuries. He recommended a minimum 40-year sentence.

The judge told the victims’ family she was sorry legal questions about the case had again brought their pain to a courtroom.

As things stand, Backstrom should remain locked up for roughly 20 more years, and then his freedom isn’t guaranteed. Decades from now, he will have to convince the state’s Indeterminate Sentence Review Board that his release would be appropriate.

Scott North: 425-339-3431; north@heraldnet.com. Twitter: @snorthnews.