It doesn't matter whether it was popular or not, it was against international law.
It's the law that is the standard, not the popularity of the crime.
It's not as clear cut as that, that's why the links you provided to show it was against international law made full use of the words 'debateable' and 'questionable'. Because the extrajudicial killing of a known terrorist who remains a threat to your country 'can be considered' self defence.
Russians killing their own traitors on UK soil aint the best comparison is it, that is against international law, has Putin been nicked for it?, any UN sanctions against him for it?