I am currently reading a book at the moment, and I got to a point were a UAV was hacked, piloted, released its pay load and crashed, could this ever happen in real life?
Not only is it possible to hack a UAV, but it has already happened. In December 2011 a US RQ-170 recon drone landed in Iran after being hijacked by Iran's security services, possibly with Russian help. This was as much poor security implementation on the drone as clever hacking on the part of Iran.
As for whether it would be possible to take over an armed drone and fire its weapons, it would certainly seem so.
Of course, everything with an external connection is vulnerable. That is the reason why always pilots should control a plane with humans. GdD already posted a well known example.
You shouldn't connect any important control-systems to an external network, this is especially important for planes, cars, nuclear power plants and so on.
As an example a really autonomous car which should improve security, should be independent from the internet and a satellite signal. For example a hacker doesn't need to hack your car, is enough to manipulate the external maps (e.g. osm, google or here) to kill a lot of people within seconds.