Ethan Crumbley, who as a 15-year-old killed four students and injured seven others during a shooting at his Michigan high school in 2021, is eligible for a sentence of life without parole, a Michigan judge ruled on Friday.

Ethan’s age and difficult home life are mitigating factors, said Judge Kwame L. Rowe. But they do not outweigh the seriousness of his crime, nor his long-term obsession with violence, the judge said, which has continued even in jail, where he used a hacked tablet to access violent content online.

“At this point, defendant’s possibility of rehabilitation is slim,” Judge Rowe said.

A formal sentencing is scheduled for Dec. 8.

The ruling followed three days of hearings in which witnesses described the callousness of Ethan’s premeditated violence and the disturbing circumstances of his childhood.

Ethan, now 17, was a sophomore at Oxford High School on Nov. 30, 2021, when he pulled out from his backpack a 9-millimeter Sig Sauer handgun. The gun’s purchase on the day after Thanksgiving had been arranged by his father in apparent violation of federal law.

The decision comes amid growing efforts to eliminate the imposition of life sentences without the possibility of parole for offenders who committed their crimes before they were 18 years old.

Twenty-seven states and the District of Columbia have banned such sentences. In Michigan, where it is not uncommon for juveniles to receive life without parole sentences, a bill pending in the Legislature would abolish the practice. A study by the Sentencing Project found that 1,465 juveniles across the country were serving life without parole in 2020.

Ethan pleaded guilty last year to 24 charges, including four counts of first-degree murder. His victims were Tate Myre, 16; Madisyn Baldwin, 17; Hana St. Juliana, 14; and Justin Shilling, 17.

As a juvenile, Ethan was entitled to a hearing, under a 2012 U.S. Supreme Court ruling in Miller v. Alabama, to determine whether he would be eligible for parole after serving at least 25 years. Such hearings are nicknamed “Miller hearings.”

At the hearing, the Oakland County prosecutor, Karen D. McDonald, had argued that he deserved life in prison without parole, which she said she normally opposes for minors.

“These acts were carried out in a cold and calculated manner,” Ms. McDonald said in her closing statement. “They were intended to have a devastating impact on as many people as he could.”

In the three days of hearings, held in July and August, testimony revealed that Ethan had planned the assault, which he hoped would be the biggest in Michigan history. On the eve of the shooting, he recorded a statement, vowing, “My name is Ethan Crumbley, age 15, and I am going to be the next school shooter.”

Despite hours of testimony describing how Ethan’s parents neglected their son — leaving him home alone as early as age 6 and ignoring his symptoms of mental illness — Judge Rowe noted on Friday that there were also happy and stable aspects to Ethan’s childhood, including vacations, pets, visits from extended family and frequent check-in text messages from his mother.

While acknowledging that Ethan’s home life was “not ideal,” Judge Rowe said, “He appeared to have a loving and supportive family.”

Colin King, a psychologist who had examined Ethan for 22 hours and was called by Ethan’s lawyer, had described Ethan as a “feral child” who had grown up abused and neglected by his parents. A neighbor reported that when he was 6, Ethan had wandered to her house during thunderstorms, saying that he was afraid, Dr. King said. Ethan lived in the midst of constant discord between his parents, including their suicide threats, Dr. King said, and the parents ignored signs that he needed help, including his claim that their house was haunted by demons.

“The part that stood out for me was when he told his parents he was hearing voices and he needed to see a therapist,” Dr. King said. “I don’t know what 15-year-old raises his hand and says, ‘My brain hurts, I need to see a therapist,’ and it never happened.”

Judge Rowe condemned the parents’ failure to procure metal health support for their son, but said he did not find the description of Ethan as a “feral child” to be accurate.

On the day of the shooting, his parents had been called to the high school after a teacher saw a drawing Ethan had made depicting a shooting. “Blood everywhere,” he wrote. “Help me.” But the Crumbleys ignored the advice of a school counselor to take Ethan home that day.

His parents have been in jail since the shootings, held on charges of involuntary manslaughter. They are the first parents in the country to face criminal charges in a school shooting, and they have pleaded not guilty. Because of the novel charges against them, the couple has filed a pre-trial appeal of their case to the Michigan Supreme Court.