Robert and Kathryn Swartz were model citizens, Catholics who ran "marriage encounter" sessions at St. Mary's Catholic Church here and who adopted all three of their children because they wanted to help youths with troubled backgrounds.

Lawrence Swartz, their handsome 17-year-old son, was a soccer star at Broadneck High School, and to all outward appearances an "average teen-ager," according to his school principal.

Today, the son they hoped would become a priest pleaded guilty in Anne Arundel Circuit Court to murdering his parents in what Judge Bruce C. Williams called "probably one of the most tragic cases that has ever occurred in this county and possibly in the state of Maryland."

Jury selection had been scheduled for the first-degree murder trial, and Larry Swartz had pleaded not guilty by reason of insanity. But at the last minute, Swartz, now 18, changed his plea to guilty of second-degree murder. Williams sentenced the youth to 12 years in prison, and recommended that the time be served at the Patuxent Institution, which treats mentally ill prisoners.

Then, in an unusual joint press conference, the state's attorney and the defense lawyers tried to explain what one defense attorney called "an insane reaction to what must have been somewhat of an insane situation" inside the Swartz home.

The press conference provided the first public glimpse into the circumstances that led to the brutal slayings, in which Robert Swartz, 52, was stabbed 17 times with a steak knife, and Kathryn Swartz, 43, was stabbed and also clubbed with a wooden maul. Kathryn Swartz's nude body was left spread-eagled in the snow next to the family's oval swimming pool.

Robert Swartz, a computer technician who had picketed an abortion clinic here every Saturday morning, and Kathryn, an English teacher at their son's high school, were strict parents who placed a premium on academic achievement, defense lawyer Ronald Baradel said.

They were, "unfortunately and tragically, exactly the wrong thing" for Larry, who ranked as "dull normal" on intelligence tests, said Baradel, a friend of the family who, like Richard M. Karceski, donated his legal services to the case.

Larry, the out-of-wedlock son of a pimp father and a teen-aged mother who abandoned him at 20 months, spent his life in a string of foster homes where he was subject to "virtually every type of child abuse that you can think of," before being adopted by the Swartzes about a decade ago, Baradel said.

Disciplined by his adoptive parents and belittled in front of others for making Cs and Ds in school, Larry Swartz had two tests at school on Jan. 16, 1984. That evening his mother, dressed in a pair of blue pajamas, and watching television in the family room, asked how he had done.

When Larry said he had done well on one test and not well on the other, his mother responded sarcastically, Baradel said.

Baradel said Larry Swartz described what happened next "[as] an explosion going off in his head."

The youth, Karceski added, was like "a balloon being pumped up and just happened to burst on this night. Larry Swartz had pent up within him as much as he could hold."

Larry Swartz remembers only "isolated instances of that night," Baradel said. "He describes himself as being up on the ceiling watching all of it . . . . He describes at one point growling like a dog or a wolf."

The next day, according to evidence summarized by state's attorney Warren B. Duckett Jr., Swartz telephoned Anne Arundel County police and reported that his parents were dead in such a calm manner that dispatchers believed it was a prank call.

Swartz, who has been at the county detention center while awaiting trial, could be transferred to Patuxent as early as tomorrow. Under the institution's parole guidelines, there is no minimum sentence he must serve before being eligible for parole.

John Riely, the nephew of Kathryn Swartz, yesterday criticized the sentence as too lenient. He said Larry Swartz "was living in a family that aspired for him to be a student and an athlete, and he aspired to be a drinker, and experiment with drugs and have an active sex life."

Riely, a Silver Spring lawyer, said his cousin was tired of restrictions and "decided he was going to put an end to that and the way to do it was to eliminate these two people."

Larry Swartz's older brother, Michael, 20, sat in the courtroom throughout the morning. Michael was estranged from the family and had been cut out of his parents' will.

Their sister, Anne Lee, 10, an orphan from Korea adopted by the Swartzes, also was not present. Anne told police she heard her father screaming on the night of the murders but when she woke Larry, he told her she was having a bad dream. Anne is being cared for by friends of the family who live on the Eastern Shore.