"Am I my brother's keeper?"

The question dating back to Cain and Abel resurfaced last week with the disclosure that David Kaczynski, suspecting his older brother, Theodore, of being the Unabomber, turned him in to the FBI. Since then, the younger Kaczynski has been lauded as an ethical role model, his decision praised as an act of selfless courage.

So when radio talk show host G. Gordon Liddy referred to him Friday not as a hero who acted in the nation's interest, but rather as a lowly "snitch" who betrayed his brother, some listeners called in steaming with disbelief.

After all, this was about the Unabomber -- the bomb-making misanthrope whose hatred of industrial society misguided him through 18 years of murder and maiming of innocent people. Could there be any other rational or moral choice for a brother -- or anyone, for that matter -- to make? Were the Unabomber your brother, you would turn him in. Wouldn't you?

"It violates the taboo against turning on one's family," argues Liddy, the convicted Watergate felon who spent time behind bars rather than turn informant. "He went out and took action which led him to believe his brother may be the Unabomber -- and then turned his brother in."

Liddy adds that if he had a brother, he would not betray him under the same circumstances. "I would have gone up there and said, Listen, I don't know if you are this guy or not. If you are, you can issue all the damn manifestos you want. But don't you hurt anybody else -- or I'll come up and whip your {butt}.' "

In morality debates, the greater good of mankind usually overrides personal considerations. And that principle applies in the Unabomber case, according to Carole Rayburn, a Silver Spring psychologist and president of the American Psychological Association's division on the psychology of religion.

"If someone is responsible for taking innocent lives, that person has to be stopped," says Rayburn. "This guy was a bomb waiting to go off -- literally. When people are sitting ducks, it clearly calls for a decision protective of life."

Rayburn, who is developing a questionnaire to distinguish between people's ingrained ethics and their situational ethics, believes that David Kaczynski acted not just out of social responsibility, but out of love for his brother as well. "He was so cautious," she says. "As I understand it, he didn't want to believe it. He was hoping above all hope that this wasn't true. But he knew what he had to do if it was, because he knew what all people need to know in the moral sense -- that life is sacred."

Rayburn says, however, that she would have done some things differently. "If it were my own brother, I would plead with him to turn himself in," she says. "If I felt that was setting him off, I would bring in a mental health team and the police. The mental health team for him, the police to protect other people from him."

Factor in the blood relationship, and some ethicists say David Kaczynski's decision to give up his brother to authorities isn't quite so clear-cut.

Responsibility to a family member does conflict with a larger responsibility to society, says James Childress, a professor of religion specializing in ethics at the University of Virginia, who knows one of the Unabomber's victims. Similar conflicts arise even when the ties aren't by blood. Spouses can't be required to testify against each other, for instance. Lawyers, physicians, therapists and ministers maintain confidential relationships with clients and patients who have committed crimes.

But "where one has evidence that a loved one is putting people at serious risk or deliberately harming people," says Childress, "then one has a moral duty to break that confidence and notify the appropriate authorities."

In "The Last Supper," a new film scheduled for release this month, five do-gooders decide to strike preemptively on behalf of the greater good, and in so doing become anything but moral.

"It's the idea that certain people not being on this Earth would make it a better place," says Stacy Title, director of the black comedy in which potential villains are invited to dinner parties, then poisoned and buried in the back yard. "It's a wishful film. What if you could kill someone whose death would make the world a better place -- like Hitler? Would you do it?"

Or, what if you could make the world a better place by turning in your brother, the Unabomber? "I would have to turn him in," says Title. "That's an easier question," because we know the Unabomber has killed. But "what if he faced the death penalty if you turned him in? You really could go nuts in this stuff."

Turning in a loved one "is a chilling prospect," says Michael Cromartie, senior fellow at the Ethics and Public Policy Center in Washington, "certainly if you know what the consequences are of murdering three people."

Cromartie says he is thankful that at a time when so many people apparently have difficulty distinguishing vice from virtue, David Kaczynski didn't. "Imagine if David had read these letters {written by his brother} and ignored them, or said, He's my brother and I can't turn him in,' and there's another 10 years and another three bodies and more maimed innocent people?"

If "stop him before he kills again" is the standard, what if he won't kill again? The Unabomber promised as much.

"It is a very moral act when you believe that this person can hurt others and may do it again," says Michael Maccoby, a District-based psychoanalyst who researches social character. "It would be a harder choice if you felt there was no chance he would ever do it again. You would wonder why you should be the one to turn him in."

Maccoby says he draws the line at clear and present danger. "If I felt my sister was dangerous to other people, if I was convinced that my patient was going to murder somebody," he says, "I have a moral obligation to do something about it."

Many ordinary people confronted with a dangerous sibling apparently come to the same conclusion. As Richard Ault, a retired investigator from the FBI's behavioral science unit now working as a private industry consultant in Manassas, puts it, "Coughing up a family member is not an unusual phenomenon" in law enforcement. During his 24 years with the FBI, betrayal by a sibling was far more common than by a parent, he says, because the denial impulses are not as strong.

"Why they do it is another question," he says. "You're really opening up a bucket of worms with that one. Sometimes it's clearly a matter of conscience and higher calling, but often the reasons are more basic: reward money, revenge, police pressure."

Motives muddy what otherwise are clear ethical choices, says Richard Shweder. A University of Chicago professor specializing in the development of conscience, guilt and morality in adults, he admits that his thoughts at hearing the news of David Kaczynski's decision weren't all positive.

"My first reaction was, What? He turned in his brother?' " says Shweder, currently a fellow at the Center for Advanced Study in the Behavioral Sciences at Stanford University. "I started imagining sibling rivalry. I imagined he must have had a competitive relationship with his brother.

"If this guy is saying to himself, I believe my brother is going to do it again and life is sacred so I've got to stop him from taking another life,' that would be a high moral motive. As opposed to, If I don't turn him in, my friends may think I'm a jerk if they ever find out.' Or, They may put me in jail for knowing and not doing anything.' "

Shweder points out that, despite the accolades David Kaczynski is getting, the public knows little about what went into the decision he made. "Are they alienated from each other or are they close?" he wonders. "A brother who has cut himself off from society may be an embarrassment for the family. You could paint a scenario in which he hated the brother for dropping out and for how he looks. You can paint many different moral dilemmas . . . and we don't know which it is."

But if the Unabomber were his own brother? Would he betray him?

"When you frame it that way, I have this sick feeling in my stomach and in my heart," says Shweder, who worries that loyalty is a forgotten value in the '90s. In some cultures, he adds, it would be a joke even to consider turning in your own brother. "I would want to confront him. I guess I would feel troubled that my loyalties should feel closer to the state than to my own family.

"If I took him at his word, that he would not do it again, and if I had only a generic suspicion he was the Unabomber, I think I could rationalize a lot of ways not to turn him in. If I had reason to believe he would kill again, I would turn him in."

Clint Van Zandt, the Fredericksburg security consultant and former FBI behavioral science expert who privately investigated the Unabomber case for David Kaczynski, finds the discussion of betrayal unsettling.

"I have a real hard time with the term betrayal,' " he says. "It has a negative connotation, and I'm afraid it could suggest David Kaczynski did something wrong when what he did was absolutely right and took a great deal of courage."

Betrayal, he goes on, "is for people who drive one or two in a car in an HOV-3 lane, or for people who dial 1-800-SNITCH hot lines. . . . What choice would David Kaczynski have had? Should he say, I'd just as soon not know the truth and put my head in the sand?' What if he did that and then a plane was blown out of the sky with 300 people and he had to wonder if it was the Unabomber and whether it was his brother?

"Each of us carries our own system of moral values," Van Zandt says. "There came a time when David Kaczynski had to look inside himself and make a decision whether to come forward.

"I think that was a decision he made under very lonely circumstances, and one he had to make himself. You and I would have done the same. I have sisters, I have sons. I know because of my 25 years in the FBI, my religious beliefs and my value system, that I would do the same." Staff writer Tamara Jones contributed to this story.

CAPTION: Gustave Dore's "The Death of Abel": A stark illustration of emotions running deeper than blood.

CAPTION: An added sting: His own brother turned in Unabomber suspect Theodore Kaczynski.

CAPTION: In this 1954 family photo, David Kaczynski, left, and his brother, Theodore, center, play in a sandbox with neighbors.