Plus the US has illegally killed US citizens, like Anwar al-Awlaki, with drone strikes. Then a few years later they killed his 16 year old son with a drone and his 8 year old daughter in a commando raid. All Americans. [1]
It’s already common for nations to do so. For individuals missiles, bullets, or bombs etc are generally more efficient as very long range drones are still expensive and cheaper options exist at short ranges.