Buckshee, I didn't mean any AI systems like Skynet or something terrorising humanity. I meant that such drone weapons are a very convenient way of dealing with unwanted political or humane opposition - and there are at least several governments out there that wouldn't hasitate to use them. Even more danger would come from the side of some sort of radical terrorists who could just release swarms of these things above a city as a kill-them-all swarm. That technology would probably make its way from military to the wrong hands relatively quick though.
There's also a problem that 90% of technology needed to make it work is already available. Tiny drones? Check. Small, high resolution cameras? Check. Facial recognition software? Check. Tiny processing units capable of efficiently running such software? Check! Small, directional charges? Bullets, duh, check. The only thing left is to interface all of these things together in a handy package and probably add some software for path tracing/selection, but that's something done in video games for very long anyway. And maps of most locations are publicly accesible anyway at Google, together with possible supplemental use of Google Street.
I kinda imagine having an opposing swarm of drones as the only solution (attacking and destroying enemy drones) since it seems like an only easily deployable and quick enough countermeasure, because it seems like building barriers around every house and public space is a really long-term investment.