It would be rather easy to attach a proper gun to the drone, with a targeting system and auto firing mechanism (already put in real world use in civilian hands:

It would be rather easy to attach a proper gun to the drone, with a targeting system and auto firing mechanism (already put in real world use in civilian hands: http://tracking-point.com/ http://tracking-point.com/ ; http://arstechnica.com/gadgets/2013/01/17000-linux-powered-rifle-brings-auto-aim-to-the-real-world/ http://arstechnica.com/gadgets/2013/01/17000-linux-powered-r...). Sure, the tech needs to be smaller, but that' won't be a problem soon. Now that's a scary assassination tool, and if properly camouflaged you can't stop it until it's too late (or drone flight is completely banned).