An execution of duty? The PM must be sure his intelligence information is better than Tony Blair's when he approves an order to kill in Syria

The killing of Reyaad Khan, a British citizen turned jihadist fighter, was never likely to provoke outrage or shock in this country. This was a youth who boasted on Twitter about being implicated in what he called “executions”, including one that he gleefully described as “the longest decapitation ever”. Anyone who so publicly delights in seeing suffering inflicted on others is not a candidate for public sympathy.

But did that give David Cameron a legal right to sanction the use of a drone to kill him? A generation ago, the question might never have been asked, but the endless controversy over the legality of Tony Blair’s decision to send troops into Iraq has heightened public awareness of the constraints of international law.

When Mr Cameron revealed to MPs that Khan and two others were killed by a British drone, he took the unusual step of adding that the Attorney General had been consulted and “was clear that there would be a clear legal basis for action in international law”. Prime ministers do not usually tell Parliament that they have consulted the Attorney General, whose advice is meant to be private. But one of the more questionable aspects of the background to the Iraq war is the changing advice Mr Blair received from his Attorney General, Lord Goldsmith. No one now thinks that “the Attorney General said it was OK” is the last word on the matter. The current Attorney General, Jeremy Wright, a barrister, is not a known authority on international law; he is, in fact, a Conservative MP who owes his promotion to Mr Cameron.

The idea that there is such a thing as international law is relatively new. Its source, for those who acknowledge that it exists, is the UN Charter, which bans member states from waging aggressive wars or from launching armed incursions on to the territory of a member state – a ban that would, in most circumstances, include the use of drones to kill people.

But the Charter does recognise that states have a right to protect themselves, including taking pre-emptive action if that is the only practical way to avoid being attacked. Mr Cameron says there was “clear evidence” that Khan, and two fighters killed alongside him, were planning terrorist outrages on British soil, and that killing them was therefore an act of self-defence. It was not, he stressed, a prelude to any wider military intervention in Syria.

There is a question of whether the drone strike violated Syrian sovereignty, which is fairly simple to answer because the Syrian government’s sovereignty does not exist in the area where Khan was killed. And, since we are told there were no civilian casualties, the action would appear to meet the requirement that the response should be proportionate to the threat.

So, we need not expect that there will ever be any credible calls for Mr Cameron to be arraigned before an international court for killing Khan. Yet he should be very careful. He is relying on intelligence reports when he asserts that these men were actually planning a terrorist attack on British soil, rather than merely boasting about it on Twitter. The definition of what constitutes an actual threat to the UK, justifying a pre-emptive strike, is elastic. There are, sadly, numerous British citizens who have been drawn or lured into travelling to Syria to join Isis. It is highly unlikely that each of them is so dangerous as to make killing them justifiable in law, even if a large section of public opinion would bid them good riddance. It is to be hoped that the extravagant praise Mr Cameron’s action has received from some quarters for this drastic intervention does not give him the idea that, in Syria, he has a licence to kill.