ON THE NIGHT of Jan. 4, 1980, Roman Welzant, 68, did with gun, kill Albert Kahl Jr., 18. That's for sure. How and why this killing came about is still in dispute, although a jury acquitted Welzant of all the charges against him -- including murder. He is, after all, an old man.

And that is the crux of the cast. Welzant, a resident of Eastwood, a Baltimore suburb, says that for something like 12 years the neighborhood teen-agers had been harrassing him and his wife -- taunting them, threatening them. Things finally came to a head the snowy night of Jan. 4, when Welzant, fed up and scared, came to the door with a gun.

What happened next is unclear. The surviving teen-agers, one of whom was wounded, testified only that they harassed the old man, throwing snowballs at his house. Welzant has a different version of events. He said the kids threatened him and that when he came to the door there was a struggle. The gun went off. One teen-ager was killed, another wounded.

For the jury, it was all too confusing, all too unclear, to return a conviction for murder. For the public, though, the situation quickly settled down into some sort of cliche -- the old versus the young. From all over the country, people sent letters of sympathy, some of them accompanied by donations. Welzant raised something like $4,000 that way. Obviously, a lot of old people have been hassled by teen-agers.

It is, of course, commonplace for teen-agers to harass old people. There is always one old person in every neighborhood who, for some reason or another, excites the imagination of kids and becomes the butt of their sometimes malicious jokes. In the Eastwood neighborhood, it was Welzant. He was called "Cameraman" by the kids because he took to photographing them so he could later identify those who were harrassing him.

It is impossible to put yourself in the position of the jury and second-guess the verdict. They heard the evidence and they reached their verdict. It is impossible also to recreate the situation that existed in that neighborhood, especially when some neighbors describe Welzant as a kindly old man while others say he was a curmudgeon who was always taking out after the kids.

None of this really matters anyway. What matters is the tendency to see both Welzant and his victim not as real people at all, but as representatives of their age groups. Welzant became a symbol of old people everywhere who either have been or fear they will be harrassed by teen-agers. As for Kahl, the dead youth, he somehow got turned into a representation of all bad kids -- the ones who treat old people like dirt.

In this particular case, this thinking has gone so far that it seemed relevant to some people that Welzant had undergone 12 years of abuse, even though the person he killed would have been only 6 when all that began. Surely, Welzant was not fed up with that one person. Surely, young Kahl could not have been after Welzant since the age of 6.

You could understand how Welzant could be at the end of his rope, exasperated, angry, frustrated. He had called the cops time and time again, and maybe his anger was justified. But what is not justified is the thinking that a member of a group should pay for the sins of other members of that same group, that it is somehow okay to hold an individual accountable for the crimes of others.

This is the basis of bigotry. The operative term is always "they" or "those people" -- "they do this and "those people do that." Welzant himself used just those words in explaining why he bought his gun 10 years ago: "Because those people's records of murder, rape and vandalism are well-documented, let me tell you."

What people? Who? Who is he talking about? Presumably it is the teen-agers. But it could be whites talking about blacks, or gentiles about Jews, or Jews about gentiles. Or it could be anytime when individuals are given the imagined characteristics of a group -- when they are held accountable for the crimes of others with similar ethnic, racial, religious or age characteristics. This, for instance, is what happened in Miami when blacks set upon whites and beat them simply because they were white.

If the people who sent Welzant checks and sided with him did so because they felt sorry for him, fine -- nothing wrong with that. But if they did so because they felt that he was right -- that the killing was justified because of the things the teen-agers have done to old people and, in particular, what had been done to Welzant -- then they were wrong and, in their thinking, no different from the Miami mob.

Welzant is just one man. When he pulled the trigger in what he says was self-defense, he didn't rid the streets of young hoodlums or make the world safe for old people. All he did was kill a kid.