It seems to me that it should be possible for a smart hobbyist to build a flying robot that recognizes people and kills them. Now, that might be a horrific idea, but not thinking about horrific ideas doesn't make them go away. What I'd like to illustrate in this blog post how a number of technologies could be brought together in a lethal fashion by someone with good computer skills.
First, flying autonomous robots. There's a tremendous amount of work been done on DIY drone aircraft (for example, there's a whole community over at DIY Drones building pacifist flying robots). The key to these is a combination of GPS and IMU. Add to that laser range finding and you've got the makings of a flying robot that can navigate indoors and outdoors.
GPS gives you location and altitude (and other sensors can give you close range altitude if you need to land on a floor, for example). GPS modules are cheap. A cheap IMU give you six axes of movement (three axes from a gyroscope giving you orientation and three axes from an accelerometer giving you acceleration). So with that you can navigate outdoors. All you need is an autopilot and those are also cheap.
Now, if you want to fly indoors then you are going to need a laser range finder so you don't bump into things. Put all that together with a quadcopter and now you've got something that can fly around indoors and navigate. Here's a little video of that.
Next you need to be able to spot people. Anyone with a recent digital camera or who's used Facebook will have noticed that it's possible to recognize people's faces quite easily. If we want to attack people then we can go with a simple face detection algorithm such as Viola Jones and it would be possible to find the middle of the person's face using a cheap camera mounted on the flying robot. Luckily, there's open source software that can do the face recognition for you.
It seems to me that it's a small step from that to shooting them.
So you could probably build a 'smart grenade' that could be tossed into a room and have it shoot people without blowing everything up. Of course, a grenade might be simpler and wouldn't be fooled by someone hiding their face from the robot.
And, of course, if I can imagine building such a device someone with time and greater resources can make this a reality. That means that the days of robots that kill us will soon be upon us.
PS Yes, this was a rather grim post. Regular readers will know that I post on a wide variety of topics (image forgery detection, Ikea train sets, Alan Turing, the Fermilab code, breaking Facebook's crypto keys, climate change and more). I can't prevent an active mind from thinking about some dark things amongst all the rest.