The only thing stopping some countries from having nuclear weapons is other countries with nuclear weapons. Maybe not all countries will have killer drones, but some probably will. Yes, in theory you could use nuclear blackmail to enforce a global ban on killer drones, for a while, but at some point someone will call the bluff. Nuclear weapons are just too blunt as a pressure tool, and military drones make too much sense to stay banned for long, being so versatile and precise in avoiding civilian casualties, especially compared to nuclear weapons.