The killings at Nahr-e-Saraj have come as a shock at a particularly difficult and sensitive time in the Afghan war. The grim fact that the British soldiers died at the hands of a supposed comrade raises questions about the key plank of the West's exit strategy: preparing the Afghans to take over their own security.
This was the second time in eight months that a member of the Afghan security forces has turned his gun on his British allies. But, realistically, there is no alternative to continuing with the policy of working alongside the Afghan forces.
It has become abundantly clear that people in Helmand, and the rest of Afghanistan, want to see their own forces – the army if not the police – replace foreign troops as soon as practicable.
For soldiers in the British Army, including the Gurkhas – who had fought alongside Afghans in an operation in the same area just the previous day – what happened yesterday was incomprehensible.
Many of them have their views about the reliability of some of their Afghan colleagues. But there is also the realisation among them that failing to adequately train the Afghans would, in effect, delay their own chances of disengaging from this war.
Just before the news came through about the killings, a group of British soldiers was saying that they had been impressed by the Afghan troops in the operation that had just taken place.
The general consensus was that that their Afghan had shown a real willingness to carry out the mission because they themselves had been in charge, as opposed to being ordered around by foreigners.
Some of the British soldiers also expressed regret that language barriers prevented them from getting to know their Afghan colleagues and suggested more should be done to facilitate better lines of communication on the ground.
Later, a few of them pointed out that – as recent events in northern England have shown – a lone gunman running amok is not an exclusively Afghan phenomenon.
However, it is also the case that yesterday's killings have shaken the trust which was being re-established after Nad-e-Ali. And, for the Taliban, who were quick to claim the credit, it has been a valuable propaganda victory, especially as the dead were the Gurkhas, who had built up a reputation for cultivating local communities.