A woman who pleaded guilty to killing the stepfather of a woman friend she had met in jail was sentenced to 50 years in prison yesterday by a D.C. Superior Court judge who said "we may never know the real reasons" for what he termed a "very, very tragic" crime.

In sentencing 36-year-old Linda D. Williams, Chief Judge H. Carl Moultrie I used the occasion to deliver a discourse on the nature of crime and the efficacy of punishment.

"There comes a time when people must be punished," Moultrie told the defendant, who stood quietly by her lawyer's side. "They may be punished for a variety of reasons, for deterrence, for reformation, for punishment.

"If we were to analyze this case and all its aspects, we would find Miss Williams perhaps fits all three categories. There is no doubt that this was a vicious crime, clearly and definitely planned."

Williams, described by her lawyer as a "sad sack," admitted earlier that she had shot Vincent A. Healy last October. Healy's body was found Oct. 11 in a closet at his Northwest Washington apartment. Prosecutors said that Williams, later arrested in Charleston, S.C., had devised an elaborate plan to conceal his death, forging letters from Healy to his stepdaughter, discussing his travel plans and telling neighbors he had gone to New York.

Williams, of Elkton, Va., had met and befriended Healy's stepdaughter, Betty Holler, while both were in jail at the Virginia Correctional Facility. Holler was serving a life sentence for her role in a contract murder. Williams, later released on probation, was staying with Healy, a security guard, at the time he was shot.

She told police she killed the 67-year-old Healy because she believed he planned to make life miserable for his stepdaughter.

In her admission, Williams said that during the last two weeks of his life Healy had become convinced that his stepdaughter was, in fact, involved in the contract murder and that he had planned to stop bringing Holler's children to visit her at the jail. Williams told law enforcement officials that she loved Holler and wanted to spare her this pain. Both prosecution and defense lawyers described Williams' relationship with Holler as "intimate."

Williams made no statement yesterday about the shooting, but thanked Judge Moultrie for considering her request to be sentenced quickly and transferred to a federal facility. Her lawyer, Nathan I. Silver, said that Williams has had a "very difficult life," and had been abandoned by her mother and sent to live with her grandparents before she was a year old.

"The life of Linda Williams has been filled with misery and discontent which culminated with the killing of Vincent Healy . . . ," Silver wrote in a memorandum to Judge Moultrie. "She appears as a sad sack, a trouble-prone and trouble-ridden bad-check artist whose crimes had a transparent authorship."

The shooting was "serious and cold-blooded," prosecutor Amy S. Berman countered in urging Moultrie to give Williams a "substantial period of incarceration."

Berman said that Williams had been arrested at least 10 times since 1969 on grand larceny and other nonviolent charges and that any "brief incarceration" had had no effect on Williams' "life of crime." The maximum sentence for second-degree murder while armed is 15 years to life.

Williams, sentenced to 14 to 50 years and fined $500, will not be eligible for parole for at least 14 years.

"It is . . . [sufficient] to say this is a very, very tragic case," said Moultrie. "Miss Williams is in the prime of her life. A person has lost his life through no fault of his own," and many other people have had "tragic moments in their lives" as a result of this crime.

Moultrie said that he had reviewed psychiatric reports on Williams and several letters sent to him about the case, but that he still could not understand the motivation for the shooting.

"Maybe we will never know the real reason," for the crime, Moultrie said.