A man convicted of torturing his girlfriend's son by forcing him to take ice baths, wrapping his head in duct tape and rubbing cayenne pepper into his eyes was sentenced to 65 years in prison for his murder.

Matthew Christenson was sentenced Friday in the 2014 death of 18-year-old Otto Smith, who had autism.

The 45-year-old Christenson was convicted in August of second-degree murder, homicide by abuse and other charges. He was also found guilty of abusing Smith's younger brother.

Superior Court Judge Chad Allred agreed to the 65-year sentence that prosecutors had recommended. It was far above the standard range of 30 to 41 years.

"Otto was a loving, kind, gentle little boy in a young man's body," senior deputy prosecutor Ben Santos said in court. "Everyone was drawn to him. He was loved, and this man took him away. And not only did he take him away, he made his last days his worst, and he destroyed many lives along the way."

Authorities said Christenson beat Smith and his younger teenage brother, made them take hourslong ice baths, sprinkled cayenne pepper into their eyes and zapped Smith with a stun gun. Smith had to wear garbage bags as diapers, and his brother had to sleep in the bathroom, authorities said.

Smith died in April 2014. Authorities said Christenson forced Smith to take an ice bath, then bound his hands and feet, gagged him with a sock and wrapped his head in layers of duct tape, causing his death. Smith's mother, Pascia Backman, found her son underneath a chair while Christenson sat nearby, drinking beer and smoking marijuana, authorities said.

Smith's death was initially ruled natural because Christenson and Backman told authorities he died in his sleep. But months later, Backman told a social worker that Christensen had killed her son and she had helped cover it up, authorities said.

Backman, a former special education teacher, pleaded guilty in November 2015 to rendering criminal assistance and was ordered to undergo mental health and substance abuse treatment.

Smith's relatives testified to the torments the brothers faced under Christenson. They said he forced hours of exercise on them, restricted their meals, made them take hourslong ice baths and beat Smith with a toy keyboard he loved.

"Imagine being a person with severe autism and developmental disabilities, already locked into a prison of your own mind; then to be beaten, abused, made to wear garbage sacks, beaten with your own toys," said Bill Carroll, an uncle of Smith's. "Everything you enjoyed was used to torture you."

Christenson's lawyer tried to dispute Backman's credibility, telling the jury that she changed her story. Christenson has said that he is innocent and that he is appealing his convictions.