After a tad more research:
In English law unlawful killing is a verdict that can be returned by an inquest in England and Wales when someone has been killed by one or several unknown persons. The verdict means that the killing was done without lawful excuse and in breach of criminal law. This includes murder, manslaughter, infanticide and causing death by dangerous driving. A verdict of unlawful killing generally leads to a police investigation, with the aim of gathering sufficient evidence to identify, charge and prosecute the culprit(s).
It is important that the inquest does not name any individual person as responsible.[1] The appropriate standard of proof is that the unlawful killing must be beyond reasonable doubt. This is when the evidence was so overwhelmingly obvious that death would result, that no other thing is taken into account. If this standard is not met, a verdict of accidental death or death by misadventure should be considered on the balance of probabilities.[1]
A verdict of unlawful killing was returned in the following notable cases:
Lt Col Rupert Thorneloe and trooper Joshua Hammond, of 2nd Royal Tank Regiment, were killed in an explosion in Helmand province on 1 July 2009.
A jury decided on 3 May 2011 that Ian Tomlinson had been unlawfully killed when he was struck with a baton and pushed to the ground by Metropolitan Police officer Simon Harwood at the G20 protests that took place in London on 1 April 2009.[2]
A jury decided on 7 April 2008 that Diana, Princess of Wales and Dodi Al-Fayed had been unlawfully killed.
The inquest into the July 7 Bombings decided on 6 May 2011 declared that the victims had been unlawfully killed.[7]
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So yeah, apparently they can't arrest the Taliban yet, but they CAN investigate them to see if they CAN arrest them.
It just seems really bizarre to me that they would declare this an "unlawful killing" when its the death of soldiers during wartime by the enemy. The fact that apparently British law doesn't distinguish between a soldier being killed by the Taliban in a firefight and someone being stabbed in a robbery is just strange.