Anyone can kill anyone and get away with it, there are literally affordable remote controlled helicopters that can be modified to carry weapons or explosive charges. Getting away with murder effectively becomes keeping yourself unlinkable to the control of the UAV, which can also be done if you have enough know how. Nothing stops you from controlling a mini helicopter half way around the world from the comfort of your home and behind a few low latency high bandwidth proxies. It becomes a game of not being traced to your helicopter, and not being identified as a target worthy of being gunned down. Of course there are defense mechanisms against such attacks as well, but only very rich and important individuals will have anti UAV security, and only an agency like the NSA would have much luck tracing the controllers.