Calvin L. Alston, the government's chief witness in the murder trial of 10 young people accused of killing Catherine L. Fuller, testified tearfully yesterday that he "did something very violent and evil" when he brutally beat the 48-year-old woman during a robbery attempt and he deserves "to be punished".

"I deserve a life sentence. . . ." said Alston, who has pleaded guilty to killing Fuller and has identified nine of the 10 defendants as taking part in Fuller's death in a Northeast garage Oct. 1, 1984. "I know it was brutal and it hurts very much."

Alston, 20, also told a D.C. Superior Court jury that he was raped in March while being held at the D.C. Jail. Defense attorneys, in a searing cross-examination, tried to use that to show that Alston was still lying about the involvement of several of those on trial in Fuller's death because some of the defendants subsequently teased Alston about the sexual assault.

Alston, appearing nervous and contentious at times during the seventh day of the trial before Judge Robert M. Scott, said that some of the defendants later had taunted him, but he denied that he now was seeking revenge.

"Weren't you harassed so badly by my client and others who are codefendants in this trial that you got so angry you cried?" asked Lillian McEwen, the lawyer for 20-year old Levy Rouse. "Didn't you plead guilty on Oct. 25 so you would not have to be transported" to trial "in the same bus" with the codefendants?

"I pleaded guilty because I was guilty," Alston replied loudly. "I faced the fact [that I should] tell the truth."

Alston pleaded guilty two weeks ago to second-degree murder and agreed to testify for the prosecution. After his testimony Thursday, some defense lawyers said Alston had delivered the most damaging testimony so far in the case, by backing up an earlier eyewitness account of another man who has pleaded guilty in Fuller's death.

Yesterday, during the second day of his testimony, defense lawyers repeatedly tried to get Alston to admit that his major motive in pleading guilty was to avoid the longer jail sentence that is carried by the charges of first-degree murder, armed robbery and kidnaping -- the charges against the 10 on trial.

"If you did wrong and you wanted to be punished, why didn't you plead to felony murder?" shouted Robert DeBerardinis, who represents Christoper D. Turner.

"I didn't want to spend the rest of my life in jail," responded a weary Alston.

In pleading guilty to second-degree murder, Alston faces a maximum prison term of 15 years to life with no mandatory minimum; the others, if convicted on all counts, would receive sentences of at least 20 years in prison before becoming eligible for parole.

In contrast to his testimony Thursday under direct examination by Assistant U.S. Attorney Jerry S. Goren, Alston appeared angry at times yesterday as he frequently clashed with defense lawyers who characterized Fuller's death as "torture" and "disgusting." Alston admitted lying to police after he was first arrested, but he did not veer from the core of his Thursday account, again identifying nine of the defendants as participants. Alston said only Felicia Ruffin, 17, did not participate.

Defense lawyers, however, attempted to show yesterday that Alston had a pattern of falsely accusing people of crimes they did not commit. Under cross-examination by Michele Roberts, Alston admitted yesterday that he had written letters exonerating one of the men he earlier had accused of raping him in March.

But Alston explained the man repeatedly had threatened to kill him and his family and that he plans to proceed with the accusations against the man in a future grand jury investigation.

Roberts also hammered away at Alston's last-minute decision to plead guilty, reminding Alston that in one letter to a court official he "put his right hand to God" in professing his innocence and had cried during one pretrial session when his lawyer urged him to accept a plea bargain.

"I cried because I was guilty," replied Alston.

Later in the afternoon, DeBerardinis returned to Alston's "right hand to God" comment when he asked Alston, "is that the same right hand you raised here in this courtroom yesterday?"