D.C. Superior Court Chief Judge Fred Ugast yesterday sentenced the last of Catherine Fuller's killers to prison, expressing "horror that something like this could happen in our city" and warning other young people that D.C. citizens will not tolerate such cruelty.

"The horror and the brutality of this crime has aroused this community and has raised the consciousness of this community in a way I haven't seen in my 13 years on the bench," Ugast told 21-year-old Calvin Alston, as he sentenced him to 12 to 36 years in prison.

Alston, who had pleaded guilty to second-degree murder as part of a plea bargain in which he testified for the government in last year's trial of 10 persons accused of killing Fuller, faced a maximum sentence of 15 years to life.

Most of those convicted of first-degree murder during that trial received prison terms of 35 years to life.

Alston, who agreed to be a government witness on the eve of that trial, told the jury that he was the person who pointed out Fuller as a possible target for the robbery that led to her death Oct. 1, 1984.

Ugast's imposition of the prison term followed a poignant plea for leniency from prosecutor Jerry Goren, who said Alston "went way beyond what he had to do" in assisting the government. Alston's trial testimony, Goren said, was of "great significance" in bringing about the convictions of eight defendants.

Eleven persons eventually were convicted of beating to death the 48-year-old Fuller in an abandoned Northeast garage during a random robbery as about two dozen people watched. It was the largest number ever convicted in a single murder in the city.

Alston was the second person arrested and at the time gave police a detailed statement that led to the arrests of eight other persons.

Although Alston was not willing to fully cooperate with the government for a long time after his arrest, Goren told the judge that he assisted the government in other ways, including secretly confirming the identity of a man incarcerated for Fuller's murder.

Goren was worried that they might have arrested the wrong man when they picked up Darryl Murchison, named by Alston in his statement, because a detective told Goren that there was another man in the neighborhood with a very similar name who had been charged with comparable acts. Goren said Alston looked at a group of photographs and confirmed the identity of Murchison, who later was released from jail after a grand jury declined to indict him.

Alston's last-minute decision to testify against the other defendants represented a great boon to prosecutors and police officers who feared that the jury might discredit the eyewitness account of another cooperating defendant, Harry Bennett, and find the remaining evidence weak. During his testimony, Alston substantially corroborated the account of Bennett, who was sentenced Tuesday to a minimum of eight years in prison.

Yesterday, Alston told the judge that "deep in my heart I'm truly sorry for what I've done." Speaking in a firm voice, he apologized to the Fuller family, represented in the courtroom by Fuller's sister, Barbara Wade. "I'm suffering but not more than they're suffering . . . .

"I understand I did something horrible . . . , " he said. "I'm not the type of person to do something like that, but it happened. I can't bring her back, but I'm sorry."