The shooting of Gayle Williams in Kabul - cruel, clinical and cowardly - reminds us why we are in Afghanistan, and should bolster our resolve to stay. It is one thing for the commander of the British Forces to say that this is an unwinnable war. It is quite another to claim that, as a consequence, the 2,000 or so aid workers like Ms Williams who risk their lives daily to improve the lives of ordinary Afghans should consider pulling out.

There are, of course, those who argue persuasively that, however well-intentioned their operations are, these people can do little to alter the course of Afghanistan's implacable history - that sooner or later, when Nato forces either give up the struggle or negotiate with the Taleban, or both, any landmarks left behind by Western agencies will be swept aside, and with them the projects that have been so expensively fostered and funded.

A school for girls, a training programme for women, a small village court for ironing out disputes, or, perhaps the work that Ms Williams was involved in - helping disabled people; all will be seen as symbols of a liberal democracy that the Taleban are sworn to eliminate. They will be first on the list for elimination when the reckoning comes, so why continue to risk the lives of the civilians who work for them? More to the point, why endanger the thousands of Afghans who draw Western wages, but cannot, like them, escape retribution?

That, however, is to ignore the moral dimension. Stated simply it is this: whose values best represent the interests of ordinary Afghans - those of the Western aid agencies working to shore up the country's infrastructure by building dams and opening up roads, or the violent extremists who shot Gayle Williams?

No one knows for certain if her killers were Taleban or not. But that there is a fudamentalist wing within that movement, backed by jihadists who stream into Afghanistan across the Pakistan border with their own agenda of intolerance, is undoubted.

My brief and limited knowledge of the country - confined to Helmand province - suggests that in those few areas where security could be guaranteed what most villagers want is restoration of the commercial life that is their life-blood. The reopening of a market brought with it a resurgence of morale, self-confidence and civic pride, always dependent on whether the conflict could be kept at arm's length. In Musa Qala, something of a frontline town, the slow and painful progress towards re-establishing basic services, such as a school, a hospital, the mosque and a decent water supply, were welcomed by the local population, not because that indicated approval of the occupying forces, but because they represented normal life.

And normal life, rather than the officious imposition of Western liberal standards, seemed to be the ambition of most of the organisations that I encountered. Those who worked for the UK Department for International Development (DfID) seemed almost over-anxious to emphasise that they were intent on “working with the grain” of Afghan life rather than attempting to change its direction.

The bearded British representative of the stabilisation unit in Musa Qala, who twice daily donned body armour and helmet to walk up the dusty road to sit and negotiate with the Governor over some small improvement in civic amenities, could not have been further removed from the imperial image of his 19th-century predecessors.

He wanted a better standard of local justice; he thought that those who worked for the Governor should be paid wages, rather than a cut of the opium trade; he had negotiated a system of justice that was open to all; he had persuaded the Governor that women deserved an education; he was attempting, very slowly, to eliminate corruption. It was an uphill task, one that might possibly, he admitted, take years to achieve. But it was, he felt, the most worthwhile job he had undertaken.

I thought of him when I heard a very different story - of what had happened in another Afghan village, where the Americans, perhaps precipitately, had established a school for Afghan girls. A brave local teacher had agreed to take it on, and, from the start, it was well attended. But the Taleban militia were watching, and when their moment came, they struck. The teacher was seized and publicly executed in a brutal and revolting manner.

I know from friends and experts that these extremists are not representative of the Taleban as a whole, and that those who represent the moderates deplore such violence. But between these random acts of savagery and the dogged commitment of Western aid workers to improving the lives of the Afghans, I know on which side I stand.

Yesterday, in Kabul, in the aftermath of Gayle Williams's death, the mood of those civilian workers for whom daily life is a constant risk, was one of anger and defiance - angry that such a devoted person had been killed because her organisation was a Christian one; defiant because they believe in what they are doing, and know how much it means to those Afghans who have been helped - the 100,000 teachers paid by the West, the hundreds of Afghan women given grants to start their own businesses, the rural development programme that has allowed farmers to market their produce across the country.

I spoke by telephone to one DfID employee who put it this way: “Why am I in this country, where the views and culture of the people are so radically different from ours? My answer lies in the Afghan staff I see working here and their families. They are Muslims and moderates, and I want to see their lives improving. And it is for the sake of people like that, that I know I can't give up.”

Those, I suspect, were precisely the sentiments that drove Gayle Williams to risk her life as she walked to work along the dusty streets of Kabul. It was, and remains, a great and worthy ambition.

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