A Suffolk judge told a Huntington Station man Friday that he deserved no mercy and sentenced him to the maximum of 25 years to life for slitting the throat of a teenage girl — a case that spurred residents to demand more police protection.

State Supreme Court Justice John Collins said the lack of emotion from Adam Saalfield, 22, throughout his trial told him something about the man convicted in August of killing his neighbor, Maggie Rosales, 18.

“In my estimation, it’s because he has all the characteristics of a sociopath,” Collins said. “And there is no hope of rehabilitation for sociopaths.”

Saalfield, who declined to speak in court, showed no reaction as the sentence was imposed.

He was convicted of second-degree murder after a trial in Riverhead that included a video of Rosales being followed, grabbed from behind on a side street off Depot Road and attacked. The killing was swift — taking less than a minute.

A trail of blood that included DNA from both Rosales and Saalfield led a quarter-mile from the crime scene to Saalfield’s home. In his closet, police found her blood on one of his boots.

The Oct. 12, 2014 killing was horrifying, the victim’s family members said in court.

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“It is not easy for me, how this person took away my daughter’s life,” said Rosales’ mother, Ignacia Gonzalez of Huntington Station, through an interpreter. “He took away happiness from my house. ... She was a girl that loved life.”

Cesar Rosales said his daughter was good-hearted, with no enemies. Friends and family have described the Walt Whitman High School senior as warm and outgoing.

“She was a good friend for everybody, all people,” he said. “She never hurt anybody. She didn’t deserve to die that way.”

He thanked police, prosecutors and others who worked to catch Saalfield. The arrest came more than two weeks after the crime, in a period that saw at least two other homicides in the Huntington Station area. In protests and at public meetings, residents insisted that Suffolk police pay more attention to their community.

Two of Rosales’ cousins also sought the maximum sentence.

“We can’t believe that we’re here,” said Kimberly Honorato, 22, of Huntington Station. “We can’t believe someone would do something so vicious to someone. He had no mercy on her.”

She and her sister, Victoria, 17, wore T-shirts emblazoned with Rosales’ face.

Victoria Honorato tearfully said Rosales was her best friend. “I loved Maggie more than anything,” she said.

“Maggie was loved,” Assistant District Attorney Raphael Pearl said. “She clearly had a bright future. “This defendant has shown not an iota of remorse.”

That’s because Saalfield still insists he is innocent, said his attorney, Craig McElwee of Hauppauge.

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“He does understand that every single arrow in this case led to him,” McElwee said.

Nevertheless, he said his client’s DNA in the blood trail did not get there as a result of killing Rosales. McElwee said he intends to appeal.