A neighborhood youth testified yesterday he overheard one of the 10 young people on trial charged with killing and robbing Catherine L. Fuller say Fuller was beaten to death because she recognized one of her assailants -- giving the jury, for the first time, a possible motive for the 48-year-old woman's slaying.

Maurice A. Thomas, 14, told a D.C. Superior Court jury he was at a store buying a soda the day Fuller was killed when he heard Timothy Catlett, 20, one of the defendants who is also known as "Snot Rag," discussing the killing with a friend.

"He said 'We had to kill her because she spotted someone we was with,' " said Thomas, who also testified that earlier that afternoon he saw at least six of the defendants punching Fuller when he glanced into a Northeast alley on on his way home from a relative's house.

Thomas said he heard Fuller yell, "Could someone please help me?" and he ran up the street.

He told the jury that when he reached home he described what he had witnessed to the first person he saw, his Aunt Barbara.

"When I told her she just told me you just don't say nothing to no one else," said Thomas, who added that a short time later his aunt sent him to the store to buy the soda.

Thomas' testimony, delivered in calm cadences, came on the ninth day of the emotional trial and provided signficant information for the prosecution's case. Until yesterday, the witnesses had testified in graphic detail how Fuller was beaten and kicked to death Oct. 1, 1984, but were unable to explain how such a brutal killing could occur over a coin purse and some jewelry.

A medical examiner ruled that Fuller, a mother of six who weighed about 99 pounds, died of blunt force injuries.

After Thomas' appearance, defense lawyers said the youth's account was important because Thomas was the first prosecution eyewitness who was not directly connected to the beating.

The three previous witneses either had pleaded guilty to charges in connection with Fuller's death or were romantically involved with one of the defendants.

Defense lawyers have used those connections to try to discredit their testimony.

"This was just some random neighborhood kid who happened to walk by when the woman was being beaten," said one defense lawyer. "He doesn't have anything to gain or lose by his testimony."

Earlier yesterday, defense lawyer Greta Van Susteren tried to portray another prosecution eyewitness, 17-year-old Carrie Eleby, as a spurned lover who would lie about the involvement of Kelvin D. Smith, because Smith was involved with another woman. Eleby testified on Tuesday that Smith, 20, was the father of her 4-month-old daughter.

"You know Kelvin Smith has a baby by another woman . . . and isn't it true that Kelvin Smith had denied he's the father of your child?" Van Susteren, who represents Smith, asked angrily, adding that the other woman and her child were living with Smith.

"It's his and he can't deny it . . . " Eleby answered. "I'm not angry because Kelvin Smith was coming to my house before he got locked up."

In contrast to some previous witnesses who often appeared hostile, Thomas remained serene under intense questioning from Assistant U.S. Attorney Jeffrey Behm and defense lawyer Frederick Sullivan.

Before Thomas took the stand, attorneys argued over what he would be able to testify about. Sullivan, Catlett's lawyer, told Judge Robert M. Scott that Thomas' statement about what he overheard could be "damaging to the point where it's almost devastating." "Absolutely," responded Judge Scott.

Although Thomas identified at least six people he saw attacking Fuller in the alley, Thomas also told the jury that he only saw two faces and could not be absolutely certain about the others because he saw their backs.

Thomas, who lived in the same neighborhood as most of the accused, said he based their identifications on his familiarity with their builds.

Thomas did not identify: Russell L. Overton, 26, Alphonso L. Harris, 23, Steven Webb, 20, and Felicia Ruffin, 17. Ruffin, the only girl, has been identified as a participant by only one witness.

Under cross-examination, Thomas repeatedly told lawyer Sullivan that although he did not see Catlett's face in the alley he recognized Catlett by a blue and white tank shirt and his short, stocky build.

An earlier witness told the jury he had seen Catlett wearing a dark-colored leather jacket in the park the day Fuller was killed.

Thomas said he did, however, see Catlett's face when he made the statement about why Fuller had to be killed.