The Five most deadly drone powers in the world.
In August 2015, a drone was used by (or on behalf of) the British government to kill a British citizen who was in Syria - BBC News 7th September. Irrespective of the legality of that killing, there ought to be serious concern that this technology will spread into the hands of unfriendly regimes (including non-State actors) with obvious consequences.
It has recently been forecast that every country will have armed drones within 10 years.
With regard to Targeted Killings conducted by the USA see Council on Foreign Relations 23rd May 2013