It was shortly before Axel Rudakubana left the house that his mother is thought to have found the discarded packaging for a knife.

His parents already knew that their 17-year-old son was ordering weapons by post; that he was watching graphic online footage of atrocities and had previously attacked a boy against whom he had a grievance. At home, his behaviour was so threatening that his own family walked on eggshells. But even though the only times their reclusive son had voluntarily left the house in the previous two years were with violence in mind, they still didn’t call the police when they realised he was gone.

Tellingly, when news began filtering out that afternoon of something terrible happening in their town, the first thought of Axel’s father, Alphonse, was whether his son might be involved. But by then, it was too late. Three little girls would never come home from their summer holiday dance workshop, and the survivors’ lives would be changed beyond recognition by what they saw. And so, in a different way, would those of the Rudakubana family.

This week, the judge leading the public inquiry into the 2024 murders of Alice da Silva Aguiar, Bebe King and Elsie Dot Stancombe in Southport concluded that lives could have been saved, had some of the many adults engaged with Rudakubana acted differently. What distinguishes his report from so many sadly similar homicide reviews before it is the uncompromising addition of his parents to the list of professionals deemed to have failed.

Judge Sir Adrian Fulford acknowledged that Alphonse Rudakubana and Laetitia Muzayire faced huge challenges as the parents of two autistic sons – the elder of good character but suffering from a neuromuscular disorder. Demonising them, he argued, would not help. (Some hope, in this political climate: Reform UK’s Robert Jenrick has already called for the Rwandan-born couple to be deported, even though they are British citizens.)

But Fulford found that in their desperation to stop their dangerous younger son being taken into care or custody, his parents lied to the authorities and to themselves: downplaying and concealing information about his escalating violence and, in Alphonse’s case, challenging some of the professionals treating his son so aggressively that his psychiatrist asked for the first time in her career to be taken off the case. Axel’s mother – who, like his father, was a survivor of the Rwandan genocide – seemed almost frozen by her fear of knives, with the judge speculating that at times she may have dissociated from events around her.

Some parents reading this may be able to understand the desperate desire not to see what is in front of you. Almost all parents will understand the fear of losing their child. But failing to make a call that might have stopped other families losing their children? That I cannot get my head around.

If Axel Rudakubana’s parents were too ashamed to admit being frightened of their own son, they probably aren’t alone. This form of domestic violence is common enough that in nearly one in five cases of women murdered by men in Britain last year, their own sons were suspects, yet it remains hidden and stigmatised. But the fatal error Alphonse and Laetitia seem to have made was in seeing only the need to protect their child, and not the increasingly pressing need to protect others outside the family from him. What happened is a living reminder that parenting can be simultaneously both selfless – that there is almost no limit to what we will suffer for our children – and selfish, where our own offspring’s interests clash with those of society as a whole.

How far should parents be held accountable, not just for their own children but for the safety of other people’s? In the US, courts are taking an increasingly uncompromising view: James and Jennifer Crumbley recently became the first parents jailed for manslaughter over a school shooting carried out by their 15-year-old son, Ethan, after a court heard they failed to get help for his deteriorating mental health and bought him a gun for Christmas. Summoned to the school to discuss a disturbing drawing he had made, they cut the meeting short because they wanted to return to work, declining to take him home. Ethan, who, unknown to everyone had a gun hidden in his backpack, returned to class and shot four teenagers dead shortly afterwards.

But if the Crumbleys were portrayed in court as distracted parents, the Rudakubana family came across more as overly enmeshed. Unable to enforce boundaries – they dared not put parental controls on their son’s devices for fear of his reaction, even after his school flagged concerns about what he was viewing online – they blamed everyone but their son for his actions. Every school will have met parents like this, who can’t accept their child is ever in the wrong, though rarely with such horrendous consequences. The question is when exactly weak parenting becomes a crime of negligence; and what public good is actually served by sending parents to jail, especially where they have other children who should not have to suffer for their siblings’ sins.

Fulford recommended the consideration of a new offence for parents or bystanders who fail to report criminal behaviour to the police. But such an offence would have to be very tightly drawn to avoid catching those who through no fault of their own see their children slipping through cracks in the system. (Axel’s doctors found no evidence of mental illness, and without a recognisable terrorist ideology, he didn’t meet the criteria for the anti-radicalisation programme Prevent either – though these have since been revised.) And even then, it’s the most vulnerable parents, who can’t afford private psychiatrists and expert assessments or navigate a byzantine maze of agencies, who would most likely bear the brunt.

One option is extending the existing offence of failing to report someone you believe is plotting a terrorist act to the plotting of mass attacks with no terrorist motive, backing that up with Prevent-style interventions for high-risk teenagers whose urge to violence isn’t driven by any recognisable ideology. If parents could seek help without feeling they were throwing their child to the wolves, we might be a step closer to preventing future Southports.

For now, however, the harsh lesson for other parents facing agonising dilemmas is that in trying to protect their own child above other people’s, Axel Rudakubana’s parents ultimately ended up losing him to a prison sentence. The rest of us can only be thankful not to find ourselves facing such a choice.

• Guardian Newsroom: Can Labour come back from the brink?
  On Thursday 30 April, join Gaby Hinsliff, Zoe Williams, Polly Toynbee and Rafael Behr as they discuss how much of a threat Labour faces from the Green party and Reform UK – and whether Keir Starmer can survive as leader. Book tickets here or at guardian.live

• Gaby Hinsliff is a Guardian columnist

• Do you have an opinion on the issues raised in this article? If you would like to submit a response of up to 300 words by email to be considered for publication in our letters section, please click here.