The only issue over which there is no argument is that they did it. Lyle and Erik Menendez have admitted that one August evening they picked up a couple of pump-action shotguns, strode through the French windows of their Beverly Hills home and shot dead their parents as they were eating blueberries and ice cream in front of the television.

As the young men blasted away, they spotted their mother, Kitty, crawling helplessly around on the floor. They went outside to collect more ammunition, reloaded and came back to finish the job. Her corpse had what coroners call a 'contact wound', and what detectives call a 'rat hole' - a bullet wound caused by holding the muzzle of the gun against the flesh. One of her eyes was blown out of its socket.

At first, the brothers made a good job of covering up everything. They removed the shell casings scattered around the living room. They went to the cinema to establish an alibi, and returned to make a hysterical tape-recorded call to the police, reporting their parents' deaths. Erik, apparently for the benefit of the neighbours, also threw a fit on the lawn, howling and rolling around on the grass in the foetal position.

A Beverly Hills policeman was so impressed by this show of grief that he neglected to do the routine chemical tests on their hands and clothing for shotgun residue. The popular theory was that Jose, their millionaire Cuban-born father who ran a video distribution company, had been the victim of a Mafia hit. He was, after all, known to be an abrasive bully with many enemies.

But then came the chilling little hints. Marzi Eisenberg, Jose's assistant, says she remembers a peculiar conversation in the limousine on the way back from a Hollywood memorial service for the Menendez parents. She later told a rapt courtroom that Lyle crossed his legs and turned to her with the words, 'Hey, Marzi, who said I couldn't fill my father's shoes?'

She took him seriously, as one might expect, given that his parents had been killed only five days earlier. 'Make your own tracks in life,' she advised him. The Ivy League student looked down at one of his tassled loafers, flexing it with his toes. 'You don't understand,' he replied coolly, 'These are my father's shoes.'

And 'the boys', as they became known, started being silly. They went on a spending spree with the insurance money. Lyle bought the Porsche that he had always wanted. Erik bought a Jeep. They put a dollars 990,000 deposit down on a penthouse. They started flaunting Rolex watches. Fatally, both confessed to Erik's therapist, who recorded the conversation. The details found their way to his estranged girlfriend, who went to the police.

The case, with its insight into the miserable life of a dysfunctional Hollywood family, has attracted a huge following in America, yet when it went to trial six months ago at Van Nuys Superior Court in Los Angeles, the outcome looked like a foregone conclusion. These young men, with their smug self-confidence (there is TV footage showing one smirking at the other in court while Judge Stanley Weisberg explains that they face the gas chamber), their expensive lawyers, private tennis coaches and Armani-type suits, appeared to be the corollary of a money-obsessed and corrupt society. Hollywood seemed finally to be spawning people who were so infected with its ghastly values that they were capable of parricide. Guilt did not appear to be an issue.

Which is why the events of last week are extraordinary. On Thursday, Judge Weisberg declared a mistrial in the case of Erik Menendez, 23, because the jury was hopelessly deadlocked after more than three weeks of deliberations. He now faces the likelihood of a second murder trial. Meanwhile, the jury in Lyle's case (the brothers each have a separate panel) is continuing discussions, which have so far been going on for 22 days.

Erik's hung jury reflects a more widespread sentiment. In recent months 'I believe Lyle and Erik' car bumper stickers have begun appearing on the Los Angeles freeways. The brothers have received thousands of letters of support. Erik's lawyer says she's receiving offers from people nationwide who want to contribute to a defence fund for her client. Almost every day, female groupies have been assembling at the courtroom to follow proceedings. As one 38-year-old businesswoman from San Diego told a reporter recently: 'I believe in them, and I wanted to stand alongside them.'

This outpouring of sympathy would not have occurred on such a scale had the brothers not been rich, young, and not unlike models from the pages of Esquire. They also invested a sizable chunk of their (now almost all spent) dollars 14m inheritance on outstanding defence lawyers, who remoulded their public images with the artfulness of a Hollywood talent agency. A couple of hard-faced spivs - who had both previously been involved in burglary - were turned into youthful Val Doonicans, with woolly jumpers and sensible, patterned shirts.

But the main reason for their rehabilitation lies elsewhere. Shortly before their trial began, after denying everything for almost four years, they admitted to the killings and to lying in the subsequent cover-up. The change of tactics came after a court ruled that the therapist's tape-recordings were admissible as evidence. Instead, the brothers mounted a new untried strategy, the equivalent of a 'battered wife' defence.

In highly emotional testimony, the brothers gave a graphic account of how they were brutally sexually abused by their father with the tacit collaboration of their mother. Few have sought to argue that Jose Menendez was anything other than a nasty, intensely ambitious man who longed to head a Kennedy-style dynasty. But the brothers painted an even more distasteful portrait.

Sobbing bitterly, Lyle, 26, testified that his father forced him to perform oral sex and sodomised him between the ages of six and eight, sometimes jabbing him with pins in the process. Erik said Jose Menendez abused him for most of his life. Both claimed that three days before the killings, Lyle confronted their father over his treatment of Erik, prompting the millionaire to respond with what they took to be a veiled threat to his life. When, on 20 August 1989, their mother ordered them not to leave the house, they were convinced their parents were about to kill them. They had been so frightened that they had earlier bought shotguns. Now they got them out.

Whether or not they are telling the truth - and their accounts of abuse were bolstered by medical experts - the fact that a jury has found it impossible to convict Erik was a huge victory for the defence. But it also demonstrates the increasing willingness of US juries to accept the notion that killing can be justified, or at least greatly mitigated, by the behaviour of the victim.

This has already been established in self-defence cases, where deadly force is considered justified if the killer is deemed to have reasonably believed that he was himself at risk of imminent death or serious bodily harm from his victim. But Judge Weisberg ruled that the brothers had not met the legal standards of 'pure self-defence'; they could have escaped their parents simply by driving off in one of their fast cars. He asked jurors to consider four options: first or second degree murder, involuntary manslaughter or voluntary manslaughter.

The ensuing deadlock suggests that a different, more disturbing, concept was in play: the belief that some types of suffering legitimise killing or maiming in order to exact revenge. The law won't protect your rights, so you must. Or so the argument goes.

This phenomenon has cropped up elsewhere. Scores rallied around Ellie Nesler, who this month was jailed for 10 years for marching into a California courtroom and shooting dead a man accused of molesting her son. Lorena Bobbitt, who is currently being tried for cutting off her sleeping husband's penis, is celebrated as a heroine by some in the feminist movement.

This is partly the fault of the judiciary itself. The system has allowed the concept of justice and vengeance to blur together, by permitting, for example, the relatives of murder victims to attend executions or to make courtroom speeches after their abusers have been convicted. It also flows from a contempt for the judicial process, nurtured by too many plea bargains.

But it does not bode well. Susan Estrich, a law professor at the University of Southern California, has summed up the situation accurately. 'The best that can be said for Lorena Bobbitt and the Menendez sons is that they were getting even. That is vigilantism. It is sugar coating for an assault on the rule of law.'

Several weeks ago, a friend who had been closely following the Menendez case told me that both the brothers' juries would be deadlocked. 'That doesn't particularly bother me,' he continued, 'Jose Menendez was a nasty man who deserved to die.' The sentiment was all the more disturbing because the speaker was a prominent Californian lawyer.

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