Mark Fitzgerald Roberts, 17-year-old stood yesterday in the D.C. Superior Court about to be sentenced for first degree murder and spoke with a cool confidence that hid both his youth and the seriousness of his offense.

He admitted he had made "a mistake in life" but denied he was any "menace to society." At one point, Roberts complained that on paper, the government charges made him look "like Al Capone," and whenever people asked him about the case, Roberts said he would reply "No, I only killed one person."

"I'm a young man. I can't benefit from 20 years to life," Roberts said in reference to the prison term he could face as Judge Alfred Burka pondered sentencing him as an adult. Because of his age, however, Roberts was also eligible to be sentenced to an indeterminate term under the provisions of the federal Youth Corrections Act. Yesterday, the decision was up to Burka.

"I know I'm young and when you're young you can learn . . . you can also adjust to society," said Roberts, whose arms gestured rapidly like a boxer's - as he addressed Burka.

If the murder had not occurred, Roberts told Burka matter of factly, he could have graduated from Ballou Senior High School, where he had been a student. And, Roberts said, the money his parents used to pay his defense lawyers could have been spent on his college education.

Roberts had pleaded guilty to shooting to death the coowner of a Chinese restaurant on South Capitol Street during an attempted robbery there in March 1977. In the courtroom yesterday sat the dead man's brother, who was shot and seriously wounded during the incident. The coowner's widow and other family were also present.

But in the months that followed the guilty plea, Roberts' lawyers argued that the youth's actions that night were the product of his constant use of the hallucinogenic drug phencyclidine (PCP) and marijuana. Roberts' stepfather wrote to Burka describing the drastic change he saw in his son's behavior when he started taking letter to Burka. He characterized drugs. Roberts himself, sent a lengthy letter to Burka. He characterized himself as "a decent teen-ager who really lost his common sense," eventually swept up by the easy availability of drugs in his Southeast Washington neighborhood.

Roberts' heavy use of PCP also concerned the D.C. Parole Board, which told Burka that bizzarre and violent behavior has often been linked to the drug. The board noted that the murder was Roberts' first offense.

Burka swayed by the arguments, initially postponed Roberts sentencing and ordered that he be examined by doctors and youth corrections officials to determine if the drugs - or its after effects - had influenced Roberts the night of the shooting.

At the hearing yesterday, Roberts' lawyer, Darrel Parker, urged Burka Burka to sentence his client under the Youth Corrections Act because he was the victim of his habitual use of marijuana and PCP."

But Assistant U.S. Attorney Alexia Morrison argued that a medical expert on the effects of PCP had examined Roberts and determined that the shooting was not a product of his use of the drug. Rather, she said, it was a carefully planned crime for which Roberts has expressed no remorse.

At one point in the lengthy hearing yesterday, Cheng Chung Chu, 37, the brother of the dead man, described the shooting at the restaurant. He spoke in Chinese and his remarks were translated for the court by a interpreter.

Business was slow, Cheng said, and he was reading a newspaper when a man walked in and announced a "stick up." Shots rang out. Cheng was hit in the groin, and his brother was killed, Cheng said. With tears in his eyes, he pointed to his brother's widow in the courtroom. Because of the shooting, Cheng said, the family has closed the restaurant.

Finally, Burka said he agreed that Roberts' drug use was not the cause of the shooting. Rather, he said, the robbery was carried out because Roberts wanted money to buy drugs.

He noted that the Superior Court probation office, the D.C. Parole Board and U.S. Parole Commission had recommended that Roberts be sentenced as an adult, indicating their belief that he could not benefit from the provisions for treatment under the Youth Corrections Act.

Burka agreed and then sentenced Roberts as an adult to serve a minimum of 20 years of life in prison for the crime.

"It is with a heavy hart that I impose this sentence that incindes more years than this defendant has lived," Burka said.