THEY MET IN HER 18th year, she and William Pratt. She had been living with her Great-Aunt Amy, a lady as old as her grandfather, a very Victorian lady who would poke her finger into young Peggy's stomach to make sure she was wearing a girdle.

Aunt Amy had plans for her great-niece. Her plans were that Peggy would go up to Wilson Teacher's College, thereby equipping herself for a profession that would allow her three summer months off a year to accompany her aunt on her travels.

"She kept her own daughter from marrying," the great-niece recalls, but gently and without rancor, "and she had the same plans for me."

And so when William Pratt came along, "tall broadshouldered" William Pratt, and offered her an engagement ring, Aunt Amy made him take it back, and forbade Peggy to ever see him again.

"The one-eyed mechanic," sniffed Great-Aunt Amy whenever she spoke to him, for William Pratt had lost an eye as a teen-ager, and was working for his father who owned a car-repair shop. And she also warned her great-niece that he was very domineering.

"I've been domineered all my life," Peggy retorted. But that was the only time she ever sassed her aunt back. "Aunt Amy, she says now, "was the family tyrant." Now, thought Peggy Pratt, now she'd be her own boss.

Anyway, two months after having met him, on a day when she really was supposed to be in teacher's college, Grace Margaret Melton eloped with William Pratt, and was married to him in Baltimore.

"I had one small moment of rebellion," Margaret Pratt says wistfully.

No. Actually, she had two. And like the first, the second act of rebellion could hardly be called small.

Because on the morning of Oct. 20, 1976 - 36 years to the month after they were married - Margarett Pratt walked into her sleeping husband's bedroom and shot him in the head.

"I saw him breathing," she says in the hesitant monotone that is her voice only when she speaks of that day. Briefly her lips quiver, but she masters them too. "A very strange sound . . . I thought - I felt - he was suffering at that point."

Then she shot him again. And by the time she went to the police and they found William Pratt a day later in the Silver Spring apartment, he was dead.

Now Peggy Pratt says that what she really meant to do was kill herself, which is why she spent the whole day after the murder contemplating suicide by the grave of her beloved dog, Junior, in the Shenandoah Valley. And that she had killed her husband because she wondered "how would he survive without me . . . because he had grown to let me handle everything. I think that's what I was thinking." And some of Peggy Pratt's friends have told her they believe she killed her husband out of compassion. And Peggy Pratt, therefore, pleaded not guilty by reason of insanity for, "Its my feeling I certainly was not functioning with my normal mind."

And the Montgomery County Jury found her guilty of second-degree murder - "malice" without premeditation. And her lawyers are appealing the case.

But one thing's for sure. This was, as Peggy Pratt says with irony in her smile, "just not a thing Peggy Pratt would do. I always got upset if I ran over a pigeon." And this is also not an instance of violent acts precipitating violend deeds. This is a passive story of repression, suppression and sludge-like despair, and - like Peggy Pratt - nothing is simple here, because she is not the sort of person who makes things simple for herself.

"You know that quotation from "Hamlet'?" She asks. "That (resolution is) 'sicklied o'er with the pale cast of thought." Well I'm that kind of person I've never done anything on impulse except eloping. And that was a mistake."

There is nothing unusual about Peggy Pratt except her intelligence. She has an IQ of 131, a revelation that emerged during the battery of tests she took at Springfield Hospital only after the murder.

Seeing her stunned expression, a psychiatrist asked, "What's the matter? Don't you believe it?"

But Peggy Pratt never thought she was extraordinary in any way. "What the hell have I done with my intelligence all these years?" was what she was thinking.

For the rest, though, she is small woman, just 5 feet and slightly plump - characteristics heightened by her prison wardrobe of jeans and a bluechecked shirt.

"I'm not 20 pounds overweight," she laughs, "I'm just four inches too short." She is . . . yes . . . amazing cheerful considering she is in the Montgomery County Detention Center, considering she will eventually go to Jessup, considering she got 15 years (seven of which were suspended) - and she will, possibly, be paroled in two years.

"I don't see why I couldn't be paroled in two years," she argues gravely, "I'm not the type of person who makes waves, who makes trouble." And there she's almost right. All her life Peggy Pratt has really never been a troublemaker(TABLE) t, "My problem was in not really asserting myself early on. Just letting myself be bored . . . "I had been thoroughly brainwashed into being what little girls are supposed to be an what wives are supposed to be."(COLUMN)Her husband loved football. She became a Redskins fan. Her husband happened to mention once