Defense lawyers attempted yesterday to portray a key prosecution witness in the murder trial of 10 young people accused of killing Catherine L. Fuller as a calculating killer, a spurned lover and a drug dealer who would lie about anything to protect himself.

During 5 1/2 hours of grueling cross-examination, Harry James Bennett, 19, admitted in D.C. Superior Court that he had lied to police when he was first arrested and subsequently had been inconsistent in some of his testimony about his own and some of the defendants' involvement in the beating death of Fuller. But Bennett, who has pleaded guilty to manslaughter and robbery in connection with Fuller's death, steadfastly maintained that he was "now telling the truth" as the seven defense lawyers turned the tables in the crowded courtroom and put Bennett's character on trial.

"Isn't it true that when you're in a tough spot you will lie to protect yourself?" lawyer Allan Palmer asked. "No, sir," replied Bennett. "But isn't that what you did on Feb. 6 when he was first arrested ?" continued Palmer. "Yes, sir," Bennett said.

On Tuesday, Bennett delivered the most damaging testimony so far in the trial, implicating nine of the defendants in Fuller's robbery and death. Bennett testified that he and about a dozen others repeatedly punched and kicked Fuller, 48, in a Northeast alley during a robbery attempt on Oct. 1, 1984, and then dragged her into an abandoned garage where the assault that led to her death continued. Prosecutors claim that a foot-long pole was thrust into Fuller's rectum during the garage attack.

Prosecutors consider Bennett's testimony, along with that of another man who has pleaded guilty to second-degree murder, their key evidence against the defendants.

Following Bennett's appearance yesterday, several defense lawyers said they believed the contradictions in Bennett's statements and testimony might lead the jury to discount his eyewitness claims.

Appearing fatigued and nervous at times, Bennett admitted yesterday that he had sold drugs and that he initially lied to police about Fuller's death to protect himself and some of his friends. He admitted he had decided to tell the police the truth only after he was arrested again in April on a cocaine distribution charge.

Bennett also acknowledged that he and Levy Rouse, accused of pushing the pole into Fuller's rectum, shared a girlfriend, and that he had heard the girlfriend was pregnant by Rouse at the time of Fuller's death. As a result, he said, he initially lied to the police and told them the girlfriend had been there and received one of Fuller's rings.

"The reason you are testifying here that Levy Rouse had something to do with that crime is because of his relationship with her , isn't it?" asked Lillian McEwen, the lawyer for Rouse. Bennett said Tuesday that Rouse was holding the pole that was thrust into Fuller.

"No, ma'am," replied Bennett, who said he had been angry with the girlfriend and not with Rouse, whom he said was a close friend.

Later, other lawyers used Bennett's admission about the girlfriend to point out that Bennett frequently changed his stories to suit his needs. In particular, most of the defense lawyers spent a great deal of time hammering away at Bennett's plea-bargain agreement with the government, under which he likely will spend less time in prison than would the defendants if convicted.

Although Bennett did not steer from his testimony implicating the nine defendants, he did make statements yesterday that differed significantly from those he made Tuesday. Notably, Bennett said Tuesday the beating had stopped when Fuller was dragged into the garage. He said yesterday he had admitted to a grand jury in April that he had punched Fuller in the garage.

"Did you hit her in the garage?" asked lawyer Palmer. "Yes, sir," answered Bennett. "So you hit her eight times" altogether, continued Palmer. "If you're counting, yes sir," answered Bennett. "We're counting," Palmer said.