I would not lose too much sleep over it, individually human being are not all that hard to kill and the world has thousands of years of practice but try in on a wider scale and you have major logistic problems. Try and target a specific individual and you have to get it within proximity and have a way to identify the target or you risk killing an innocent bystander. Then the attacker may be surprised at how an individual may respond to it, how fast they can move, at what range they can hear/see the gadget and what they can do to deflect it.
Pick up a dinner place (if you are eating), a book if you are reading, duck behind your laptop screen and if you have any idea of what is coming, a tennis racquet would put one out of action real fast. At the passive level a mosquito net would be no fun for the propellers. On a large scale the delivery of a large number of the mini drones would be a problem as anything more powerful than a starving third would country has its own air force and even if its a 50 year old Mig 21 doing the take down, it would have no problems in whacking anything big enough to carry and release thousands of minidrones.
If you regularly look at junk on Youtube you will get to see all sorts of speculative bullsh*t about new and improved gadgets for sale to the military/police on how to kill or control large numbers of people but if the chyte really hit the fan in terms of conflict, it would be reliable old fashioned guns and explosives, tanks, aircrafts and if anyone of these morons are game enough, the end of the world (for humans) with flying nukes. :P