The sentencing hearing of the teenager who killed three young girls and wounded 10 other people last summer in a knife attack on a dance class in Southport, England, began on Thursday.

Judge Julian Goose, who is presiding over the case, told the attacker, Axel Rudakubana, 18, that a life sentence would be inevitable after he pleaded guilty on Monday.

Mr. Rudakubana appeared in Liverpool Crown Court wearing a gray sweatsuit, with a blue medical mask covering his mouth and nose. When asked by the judge to confirm his name, he refused to speak and silently put his head in his lap.

But shortly after the sentencing hearing began, as prosecutors were reading out the details of the case, Mr. Rudakubana began screaming from the defendant’s dock in the back of the room, “I need to speak to a paramedic because I feel ill.”

The judge noted that medical specialists had examined Mr. Rudakubana that morning and determined him fit to attend the hearing. His lawyer told the judge the defendant had not eaten for a number of days, and Mr. Rudakubana continued to yell for several minutes.

Judge Goose said: “These proceedings are being conducted under my control, not yours, Mr. Rudakubana. Do you understand?” He then ordered Mr. Rudakubana removed from the court, saying, “I won’t have him disrupting.”

Before Mr. Rudakubana’s sentencing on Thursday, prosecutors read out the details of their case against him, revealing the harrowing nature of the attack on July 29 which Deanna Heer, a lawyer for the prosecution, said “targeted the youngest, most vulnerable in order to spread the greatest level of fear and outrage, which he succeeded in doing.”

She told the courtroom that while Mr. Rudakubana was under arrest at the police station after the attack, he was heard to say, “It’s a good thing those children are dead” and “I’m so glad.’”

Ms. Heer recounted how he had traveled by taxi to Hart Space, where a sold-out Taylor Swift-themed dance class for children ages 6 to 11 was underway during the summer break from school.

Visual evidence shown in court, taken from CCTV footage and police-worn body cameras at the scene, showed Mr. Rudakubana arriving outside the dance studio that was crowded with 26 children.

He entered the building and rampaged through the room, stabbing several children as well as Leanne Lucas, who had organized the class. Moments later, screams could be heard on outdoor CCTV footage, before children began running from the building.

Some were covered in blood and collapsed before bystanders came to their aid. At one point, a dance teacher who had shielded one of the young girls in a bathroom was seen being helped from the room by the police.

Several people wept in the courtroom as the footage was shown, and several chose to leave, overcome by emotion.

In the attack, the injuries suffered by Bebe King, 6, and Elsie Dot Stancombe, 7, were so severe that they died inside the building, the police said. Alice da Silva Aguiar, 9, ran outside with the other children but soon collapsed. She was taken to the hospital and died the next day. Eight other children and two adults were wounded in the attack.

Since Mr. Rudakubana pleaded guilty on Monday, a portrait of a deeply troubled young man obsessed with violence has emerged, as has the fact that he was on the radar of the local authorities for years before the July 29 knife attack in Southport, a town north of Liverpool.

After the attack, Britain was rattled by a series of riots as disinformation about the attacker’s identity swirled on social media and messaging apps. False claims that he was an undocumented immigrant or newly arrived asylum seeker were amplified by far-right agitators. Mr. Rudakubana is a British citizen who was born in Wales to parents originally from Rwanda.

There was no evidence that he ascribed to any particular political or religious ideology, the police and prosecutors said.

At age 13 and 14, he was referred three times to Prevent, a British counterterrorism program, because of his fixation on violence, but those referrals were ultimately dropped because it was determined each time that he did not meet the threshold for intervention.

Prime Minister Keir Starmer said from Downing Street on Tuesday that the attack was a sign that terrorism in the country was evolving, and that young people were being radicalized by “a tidal wave of violence freely available online.”

“We also see acts of extreme violence perpetrated by loners, misfits, young men in their bedroom, accessing all manner of material online, desperate for notoriety,” Mr. Starmer said, noting that some became “fixated on that extreme violence, seemingly for its own sake.”

Mr. Rudakubana was also convicted of a weapons charge for possession of the knife used in the attack, for production of a biological toxin and for “possessing information” described as “of a kind likely to be useful to a person committing or preparing an act of terrorism” after investigators found ricin, a lethal toxin, and a PDF file titled “Military Studies in the Jihad Against the Tyrants: The Al Qaeda Training Manual” in his home.

The judge will not be able to sentence him to prison for life without parole, because he was 17 at the time of the attack.

In 2019, Mr. Rudakubana was expelled after he brought a knife to school and a few months later he returned to attack a student with a hockey stick. He was then enrolled in a school for children with specialized needs.

He had been withdrawn from his family and the community long before the attack, and barely left home.

A week before the attack, Mr. Rudakubana tried to travel to his former high school, the police said, but his father ran out of the house and pleaded with the taxi driver not to take him. Eventually, the teenager returned to the house.

The case has raised questions about how the authorities may have missed opportunities to stop the violence before it began. The government has said it will conduct a public inquiry into the case to better understand what happened and what needs to change. But the case has also highlighted the issue of young people fixated on extreme violence who gain access to online images and messages that drive that obsession.