http://www.bbc.com/news/magazine-34184856
Who, What, Why
The Magazine answers the questions behind the news
8 September 2015
From the section Magazine

Who, What, Why: When is it legal to kill your own citizens? Two British jihadists were killed by an RAF drone strike in Syria.

For a related article, see, e.g.,
Govern, Kevin H., Operation Neptune Spear: Was Killing Bin Laden a Legitimate Military Objective? (Chapter 13) (April 1, 2012). Targeted Killings Law and Morality in an Asymmetrical World, Editors: Claire Finkelstein, Jens Ohlin, and Andrew Altman (Oxford University Press, 2012). Available at SSRN:http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2083584