Mercy most just to Tania Clarence

In his sentencing remarks at the Old Bailey, Mr Justice Sweeney addressed the defendant thus: "Tania Clarence, you are now aged 43, and are of previous good character."

That statement had a depth charge of unimaginable sorrow because this was no ordinary criminal. For life had dealt Tania terrible blows.

After her daughter, Taya, was born in 2006, Tania suffered several miscarriages before giving birth to Olivia in June 2009. The happiness of Tania and her husband, Gary, seemed complete when, soon after, she fell pregnant with twin boys.

Max and Ben were born prematurely at 26 weeks while the family was on holiday in Portugal in July 2010. The babies stayed in intensive care for four months and returned to the Clarences' Wandsworth home in November.

That's a hell of a strain for any parent. In August 2010, when the twins were still in hospital, it was discovered that Olivia suffered from spinal muscular atrophy type 2, or "floppy baby" syndrome - a genetic disorder that leads to muscle-wasting and considerably reduced life expectancy. Three months later, the Clarences were told Max and Ben also had SMA.

Tania , who had suffered depressive episodes throughout her life and had a family history of suicide, had to deal with four children under the age of four, three of whom had a life-limiting condition that meant they would never feed themselves or control their movements.

All the evidence suggests Gary and Tania carried their beloved burden with indefatigable courage. They moved to Surrey and borrowed heavily to equip their new house with everything to make the children's lives as easy as possible. They put quality of life for their babies before operations and painful medical interventions, which they deemed unnecessary, given the circumstances. The professionals disagreed. There were clashes with doctors and social services. In one crisis, the Clarences were unfairly accused of tampering with Olivia's medical equipment. Then, Tania's rock, social worker Suzie Holley, was replaced with a novice because social services decided Holley had got "too close" to Tania.

Imagine what a blow that was to a woman struggling to cope day-to-day while her brain was still clanging with the death sentence her three babies were living under.

On Easter Tuesday, while Gary was away with their oldest child, Tania took a nappy and smothered Ben and Max and then Olivia, before attempting suicide. In a note for their nanny, she said she had to kill the children and herself because Gary would never cope with them on his own.

It's the kind of warped maternal logic - love gone mad - which makes sense to someone who is mentally ill. In a note to her husband, Tania said the only thing giving her the strength to kill "Liv" was the thought that Max and Ben were already playing in Heaven as they had never been able to play before.

It is, as the judge remarked, the saddest story he had ever come across. And yet, even amid the horror, there are small things for which to be grateful.

There was no witchhunt of Tania. A public, better educated about depression than it was a few years ago, grasped from the start that a doting mother who had done that to her children was to be pitied, not reviled. The prosecution accepted her plea of guilty to manslaughter on the grounds of diminished responsibility, due to being mentally ill. Sweeney, who could have sent her to a prison, elected instead for the humane option of a hospital order, and to have Tania confined for her own safety under the Mental Health Act.

Questions were also raised about the culpability of the authorities who were supposed to be supporting the family, not adding to their woes. The judge noted that Tania had barely slept in the months before she killed her children because of the "increasing distress" of the children's treatment by doctors.

The quality of mercy was palpable throughout the case. As the judge told the sobbing woman in the dock: "The prosecution accepts you loved all four of your children - indeed, there is a substantial body of evidence that they were happy and well looked after - and that you were grief-stricken that Olivia, Max and Ben were destined to die before you."

Compare this with the horrendous treatment that befell Sally Clark, also of previously good character, who in 1999 was hounded and wrongfully jailed for the "murder" of her two cot death babies. Her depression was mistaken for heartlessness, her inability to feel for indifference.

Gary has said he will stand by Tania on "the long road ahead". He is clear she loved the children but was "overwhelmed by depression because of constant pressures of caring for them".

Gary says lessons need to be learned from Tania's "dedication and love which turned to despair and utter hopelessness". One lesson is for experts in disabled children who think they know better than parents who care for those children. The other is that mental illness is serious, and our society is more civilised for understanding that.

The traditional business of the law is justice balanced with mercy. In the case of Tania, however, there was no need for balance. Mercy was the most just.