A D.C. Superior Court jury yesterday convicted David L. Garris of first degree murder for the killing of Kathleen Boyden, an advertising sales representative at WMAL radio bludgeoned to death last September in her northwest Washington apartment. Garris worked as the janitor at the building.

Garris, 33, of 1812 Vernon St. NW, was also convicted of six other charges, including robbery and grand larceny. Judge Fred B. Ugast set sentencing for Feb. 11. Garris faces from 20 years to life imprisonment on the murder charge alone.

He stood in stony silence as the jury rendered its verdict. Only three days earlier, he had taken the stand in his own defense and testified that he bar-hopped, met with friends and eventually returned home to his wife on the night Boyden was killed.

"You may have opinions about Mr. Garris and his lifestyle," argued Garris' defense attorney Christopher G. Hoge to the jury as the two-week trial ended. "You may not like Mr. Garris very much . . . does that really mean he's guilty of murder?"

After the jury verdict was announced at 4:30 p.m., Hoge said that he probably would appeal the case and that Garris felt "acute disappointment with the verdict."

Boyden's decomposing body was found Sept. 23 in the rear of her car, which was parked in the 3400 block of 17th St. NW in the Mt. Pleasant area of the city. Several days earlier, she had been reported missing by coworkers and police found traces of blood in her apartment.

According to testimony in the trial, Boyden was last seen alive Sept. 17, when she returned home to her apartment at Idaho Terrace, 3040 Idaho Ave. NW. Moments before she went to her apartment, according to a desk clerk who testified at the trial, Garris had picked up a set of Bowden's keys.

The next morning, according to testimony from Garris' former girlfriend, Rene Carter, Garris gave her a bloodied shirt and other property later identified as belonging to Boyden.

Assistant U.S. Attorney Richard C. Otto told the jury Thursday that Garris' crime had been a "cold, calculated, well-planned and meticuously executed criminal exercise," and he said that Garris' alibi was flimsy at best.

The government's evidence alleged that Garris had used the key to enter Bowden's apartment before she returned and manipulated the window so that he could come back later. Otto said that after Boyden returned to the apartment and retired for the night, Garris entered the apartment, allegedly stole an $1800 ring and other property and then bludgeoned her to death with a hard object after she awoke and discovered the intruder.

Otto said that Garris then "embarked on the next phase of his criminal plan, an exhaustive series of attempts to dispose of every bit of that crime . . . including that woman's body." Garris allegedly took Boyden's body out of the apartment building in the early morning hours and later left it in her parked car two miles away.

When police later entered the apartment, Otto said, they found that someone had cleaned up, even turning around the trash can so blood spots were less obvious.

Otto told the jury that believing Garris' alibi would mean disbelieving at least nine government witnesses.

Garris denied killing Boyden. He testified that on the night of the slaying, he went to a roller rink, the home of a female friend, a Gloria Avenue nightclub where he played pinball and drank beer, and a sandwich shop near 18th Street and Columbia Road. He testified that he returned home at 3 a.m., where his wife braided his hair and he then fell asleep.