Such atrocities, after all, are often waged for attention and carried out in the name of political or religious goals, and a trial gives perpetrators more of what they crave: a huge audience.
In the worst case, there is a risk that Breivik's trial, during which he has raised a right-wing salute and gloated over his killing rampage, could spawn copycat crimes by others who share his hatred of Muslims.
"There is a contagion effect that one has to take into consideration," said Brigitte Nacos, a Columbia University professor who studies terrorism and the mass media.
Col. Zbigniew Muszynski, the head of Poland's Counterterrorism Center, said security experts believe there have already been violent acts inspired by Breivik, including an Italian extremist's killing of two African immigrants in Florence late last year.
"There is always the potential danger that someone who hasn't been exposed to extremist propaganda could become interested in it and could undertake activity that is in violation of the law," Muszynski said.
Democracies generally tend to give suspects in even the most horrific crimes the chance to speak out freely in court, though sometimes they place limits on what can be broadcast to a wider public.
When Serbian autocrat Slobodan Milosevic went on trial in The Hague on war crimes charges, he tried to use the court as a pulpit to defend policies that led to the Balkan wars of the 1990s and to rouse nationalists back home.
The court closed his sessions several times when his rhetoric reached fever pitch, though by then - with Milosevic's rivals holding power in Serbia - his words had little impact on the political situation in his homeland.
Peruvian courts silenced Abimael Guzman, the founder of the once-fearsome Shining Path guerrilla group, during a series of trials after his 1992 capture.
His sessions were mostly held behind closed doors with journalists watching from a soundproof booth. Microphones were shut off during a 2004 trial, when he was given a chance to speak, and used it to declare: "Long live the Communist Party of Peru! Glory to Marxism-Leninism-Maoism! Glory to the Peruvian people!"
In Breivik's case, there's a strong sense among Norwegians that the public has a right to know as much as possible about the killing rampage.
The court has allowed reporters to cover the grisly details of Breivik's testimony over the past week-and-a-half. But it has also tried to deny Breivik excessive publicity: Photos for the most part are allowed only at the start of sessions, and filming during his testimony has been mostly restricted.
Even so, some Europeans say their gut reaction is that it's too much coverage. They have been horrified by images splashed across newspapers of Breivik making a defiant fisted salute and reports of his bloodcurdling admission of his crimes.
"The trial should have been kept secret or at least behind closed doors and the court should only issue a statement with the verdict," said Malgorzata Rogala, a 50-year-old translator in Poland, where courts sometimes limit media coverage in sensitive cases. "It is an insult to the families of the victims and publicity he does not deserve."
But some legal scholars argue that true openness is essential, if only to prevent conspiracy theories. And public trials are a bedrock of democracy that experts say must be allowed in most cases.
"The underlying human rights dimension of this is that public trials have a very central place in the consciousness of the community," said Mike Newton, a law professor at Vanderbilt University. "And so the presumption should be an open trial unless there is clear evidence that what goes on in the courtroom is just an extension of hostilities."
Newton, co-author of "Enemy of the State: The Trial and Execution of Saddam Hussein," argues that Saddam's trial is a case where there would indeed have been good argument for a closed trial because the deposed Iraqi dictator used his testimony - broadcast on Iraqi television - to encourage the ongoing insurgency.
Perhaps ironically, some extremists who carry out horrific attacks to make a political point reject the opportunity to speak out in court.
An example is Timothy McVeigh, who acted out of hatred for the U.S. government when he bombed a federal building in Oklahoma City in 1995, killing 168 people. Defense attorneys feared that if McVeigh testified, it would open him up to too many questions from prosecutors about his role in the bombing.
McVeigh's distrust of the government might have also played a role in his refusal to testify, but he later worked with a biographer to tell his side of the story, and he appeared to welcome the attention he got before he was executed in 2001.
"Timothy McVeigh got a lot of mail when he was on death row. Some people were supportive, and he even got marriage proposals," Nacos said. "McVeigh died a happy man. He said he accomplished what he wanted."
Cult leader Charles Manson, who persuaded others to kill for him, also refused to testify in the raucous 1970 trial of him and other members of his "Manson Family."
In an unusual proceeding, Manson was allowed to testify outside the jury's presence so that the judge could rule on whether his testimony was admissible in front of jurors. Manson spoke at length, telling his life story and denying killing or ordering anyone killed.
But when the judge said he could tell his story to the jury, he refused.
Court testimony is clearly not a criminal's only chance at attention.
Breivik shared his vision with the world in a 1,500-page manifesto that he posted online before waging his attacks on July 22 - a bomb in Oslo's government district followed by a shooting spree at the governing Labor Party's youth camp outside the capital.
Manson exerted a fascination on some simply due to his bizarrely charismatic personality and his involvement with celebrities.
Unabomber Ted Kaczynski - who also never testified in his trial - got the New York Times and the Washington Post to publish his rambling manifesto in 1995.
Terrorism expert Beatrice de Graaf argues that the Norwegian court has struck the right balance in the Breivik case.
De Graaf, who is leading a research project called "Terrorists on Trial and the Courtroom as a Stage" at the International Center for Counterterrorism in The Hague, says the trial of Breivik, who has confessed to the massacres, can be seen to some extent as a "show trial" given the huge attention surrounding it.
But allowing the trial to be a stage, she argued, can be cathartic: It gives victims a chance to voice their pain, hear why the perpetrator carried out the act, and may also serve to educate society.
As an example, de Graaf - a professor at Leiden University in the Netherlands - cited the Nuremberg trials of Nazi leaders after World War II that forced German society to acknowledge the scope of Nazi crimes.
"You cannot withhold the press from covering such trials," said de Graaf. "The point isn't the show, because there will be a show. It's what end the show serves."
She said that allowing only written coverage of Breivik's testimony - but not film footage - is a "very clever solution" given the importance film and photos can have in recruitment and brainwashing of extremists.
It informs people of his twisted views while depriving him of the chance to appeal emotionally to other right-wing extremists.
"If you would give him the stage before cameras," she said, "it would be exactly what Breivik was after."
By VANESSA GERA
Associated Press