Image recognition and flight control makes a drone, but probably not a killer drone, as that would presumably require some sort of munitions - like a gun or explosives. So it might be efficient enough to just limit access to those parts?
My background is an engineering role in the military and my current path is CS. I don't think it's unfeasible that an individual could build a crude drone that would kill a single person. Building an entire swarm of death robots is a different story.