Capturing him would seem a lot more beneficial use of JJs life than taking him out as had been done. Militarily he was nothing and the risk he himself posed to British (if we have to refer to 'our' forces) lives was minimal.
From what little I've read on his death he was approaching Raqqa in a vehicle. How do you propose getting a force to Raqqa, bearing in mind it is a major HQ for IS, and do you think it safe to send a kidnap squad, sorry arresting officer, in to a place like that?
I would prefer capture, questioning and trial too but I don't think JJ was that spotty teenager guilty of anti social behaviour on the local village green.