All too easy. Recall the kid about a year ago that mounted a pistol on his drone. Drones and self-driving cars are all too likely going to be terrorist tools. A grenade seems easy to envision as it could be held in place by the pin mounted on a servo motor. Active to remove it and the grenade is armed and dropped, drone returns for reload. Be hard to top $1 billion of damage with one grenade.
The Shadow had the equivalent of "flying bombs" aka "cruise missiles" - controlled from a mother plane.