A D.C. Superior Court judge sentenced Sylvester R. King Jr. yesterday to a prison term of 79 years to life for killing a District police sergeant, saying King had forfeited "his right to live in this society."

Chief Judge H. Carl Moultrie I, in pronouncing sentence, said that King "has proven by his previous action, as well as the action in this case, that he no longer observes the folkways and the mores of this community . . . . There's nothing this court could do . . . that could bring back the life of Sgt. Joseph Cournoyer . . . but this court intends to punish King accordingly."

King was convicted in August of first-degree murder in the shooting of Cournoyer, a six-year member of the police force. Cournoyer was shot Jan. 29 at the Minnesota Avenue Metro station shortly after King and two other men robbed a nearby meat store. Cournoyer was shot through the heart after escorting King off a Metrobus.

Prosecuting attorney Alan Strasser said when King fired a shot at Cournoyer, he fired a shot at every police officer in the city. Strasser urged Moultrie to give King the maximum sentences for the seven charges on which he was found guilty, including robbery and assaulting a police officer.

King showed no emotion as Judge Moultrie announced somewhat lesser sentences that totaled 79 years to life and will run consecutively, but a discernible sigh could be heard in the courtroom crowded with police officers and Cournoyer's widow and family.

King, who may not be eligible for parole for 79 years, did not testify at his trial, but his attorney tried to repudiate a confession that King gave police after his arrest. In the videotaped confession played during the trial, King told detectives that the shooting was an accident and that "it was him or me."

Charles A. Blackwell and David A. Corbin, who also participated in the robbery, have pleaded guilty to second-degree murder in Cournoyer's death.

Yesterday, King, dressed in a beige three-piece suit, made no statement in court, saying he had written Moultrie a letter. King's lawyer, Richard Greenlee, said King had told the judge that he expected to be incarcerated but asked to be sentenced to Lorton Reformatory, the city's prison in the Virginia suburbs, because his mother is old and very ill.

Judge Moultrie told King he would recommend the Lorton placement but told King that he should have considered before he committed the crime the suffering his actions would cause.

After the sentencing, some of Cournoyer's colleagues on the police force said they were pleased with the prison term, but added that they hoped for a change in District law that would allow stiffer sentences for people convicted of killing police officers.

U.S. Attorney Joseph E. diGenova said King's case highlighted the need for a special statute in the District that impose a mandatory life sentence without parole for killing a police officer. Currently, there is no District law dealing exclusively with killing a police officer.