The state's monopoly on violence and Litvinenko.
Killing Reyaad Khan, a jihadi halfwit who allegedly was plotting attacks on the UK despite seemingly no one being arrested in connection with those plots, was fully justified under international law (PDF), essentially because in the classic formulation of Jimbo and Ned, HE WAS COMING RIGHT FOR US.
International law is remarkably malleable. A state making a decision on how and when to kill its enemies can nearly always be justified in some way. The murder of Alexander Litvinenko was outrageous and despicable, but hardly unforeseeable given whom he was working for. MI6, as usual, failed to foresee it, and as usual, continues to so much as confirm or deny if he was an agent. The evidence it did provide to the inquiry, which itself only got going after Marina Litvinenko challenged the home secretary's decision not to grant one, seems to have led Sir Robert Owen to point the finger of blame directly at Putin. Rather a shame they didn't reckon on the threat the Russian state considered him to be before he was slipped a polonium nightcap.