You’re talking science fiction now. If it becomes so small as a fly how on earth would it be energy sufficient? With a battery? And that battery will hold enough energy not only to keep it flying but also to provide for a taser that could incapacitate an individual? And at that size it should also carry some communication infrastructure to receive instructions from a command center, a gps, and a cpu capable of running face recognition or even detect movement patterns because otherwise you can’t have efficient crowd control. Sure, in a couple of generations we’ll get there, but not in the immediate future.
