Aren't assassinations targeted by definition? And surely this kind of thing is already doable if you're motivated enough, e.g. every smart phone is both an identity mechanism and a signal (thanks to all of the tracking/location systems) that could probably be used to guide a weapon system to an intended target. Hell, there was a kiwi engineer who built a cruise missile in his garage almost 20 years ago. I don't know that a killbot system offers much of an improvement over existing methods for the difficulty of execution, with the probable exception of, say, air superiority where the capability of autonomous systems will far surpass human ability. But even there we're probably two generations away from fully autonomous systems; the next generation seems to be robotic wingmen.

What really stops the average person from building these systems isn't necessarily the difficulty of the system itself, but rather the impossibility of getting the components. The drone is just a delivery mechanism for the ordinance, and good luck getting your hands on that.

Also, for said killbots to be anything other than a trivial threat (just jam the signal) they would need pretty decent onboard AI to function independently, which I guess is the point they're trying to make. But there are almost certainly far easier and more cost effective ways to get the same effect that don't rely on currently non-existent technology.