The government has urged South Africans like Behr to tell their stories, saying the country cannot move forward until its people fully understand what went wrong in the past.
But the overwhelming criticism that has greeted Behr's confession raises questions about how much truth his country can stand.
"I've become a symbol of everything that was ever wrong in South Africa - of betrayal, of false beliefs, of complete and utter culpability," Behr said in a recent telephone interview from Oslo, Norway, where, every summer for the past five years, he has taught a university graduate course on peacemaking.
Behr appeared at a writers' seminar in Cape Town in early July to reveal that while he was leading protests against white minority rule as a university student, he was reporting on his comrades to the police.
South Africans have been urged to tell such stories. The new black-led government established a Truth and Reconciliation Commission as a forum for soul-baring, saying the country could not move forward until its people fully understood what had gone wrong in the past.
Behr, product of a conservative, white, middle class family, said he was willing to believe that "apartheid was okay" as a 22-year-old in 1986. That was the year a relative who was a high ranking police officer first asked him to report on the activities of the few leftist students at the University of Stellenbosch. The money the white government offered helped pay tuition.
Within a year, he said, the young people he was spying on had convinced him their cause was just. But he continued working for the police, not so much for the money, he says, but because he was afraid of being exposed as a traitor. His handlers also threatened to reveal that he was homosexual, something Behr no longer hides but which was very much a shameful secret in 1980s South Africa.
By 1990, the government had embarked on negotiations that led to historic all-race relations in 1994, and its security forces lost interest in Behr.
He kept his secret until he began to fear that the attention being showered on his first novel, "The Smell of Apples" - including two national literary awards - would create new interest in his personal story, and lead to his being exposed.
His confession has been far from welcomed as a contribution to national healing. Behr has been accused of seeking publicity, of being insincere and manipulative, of providing too few details - even of being too remorseful.
"He apologises for the betrayals, for his motivation, for his lack of moral courage; he apologises for apologising; and then, in an infinite regress, he apologises for apologising for apologising," Nic Borain, who worked with Behr as a student activist, wrote in the Weekly Mail and Guardian days after Behr's speech.
Similar denunciations came from other colleagues and friends, appearing in newspapers across the country. Most acknowledge that Behr committed no crime, and probably provided little information to which the state did not already have access. But the outcry has been harsher than the reaction to former soldiers and cops who, hoping for amnesty, have gone to the Truth Commission to tell of committing murder and torture in the service of apartheid.
While much of what the police and military officers have said was already widely known in South Africa, Behr is perhaps the most prominent figure so far to come forward with surprising revelations.
Pearlie Joubert, who protested alongside Behr as a student, said she can at least understand white rightists who acted out of conviction, no matter how terrible their crimes. Behr's personal betrayal, and his acknowledgment that he continued spying even after he began to realise that apartheid was wrong, seems to hurt the most.
Nonetheless, "I will have to forgive Mark," Joubert said, accepting the links South Africans are trying to forge between truth, reconciliation, and progress.
"Only if we know what happened can we know how did it happen, how did it start, where did the rot set in," she said.
Jane Taylor, who organised the Cape Town writers' conference at which Behr made his confession, said his very ordinariness may be part of why South Africans are finding it difficult to forgive. Many would like to believe apartheid was a crime committed by the few, not a way of life for most whites.
Taylor said she had no warning Behr planned such a startling confession at her conference. He was invited because even before telling his spy tale, he had made something of a career of admitting to, and apologising for, having once supported apartheid. "The Smell of Apples", his 1995 novel, was lauded as a revealing, pseudo-autobiographical account of the twisted logic of racism.
At the University of Oslo, Behr said he opens each of his summer courses by presenting himself as an example of how an ordinary person can slip down the path of evil.
For the first time this summer, he also told his students of his spying. The class includes Israelis, Palestinians, Bosnians - all with their own horrors and private guilt, and all, Behr said, intrigued by his forthrightedness.
After Norway, Behr will go to the United States to study literature at Notre Dame. He was unsure when he would return to South Africa, sounding saddened and frightened at the storm he's caused back home.
He was eager to know if anyone has spoken out in support, if the anger was dying down, if his countrymen were ready to talk.
"I say in my speech that this is the beginning of my process of going public," he said. "From here, as I say in my speech, the interrogation must begin."