Someone attached a handgun to a drone. You can get off one shot at a victim not expecting such an attack. Probably the same with a small IED.
I spent Wednesday night watching BattleBots with my brother. The rules now allow several independently piloted vehicles so long as the combined mass falls within the limit. Some teams attempted to use a drone. None were effective.
Drones are probably most effective as a terror weapon against those who do not know the limitations of the device.