Where this stuff does get mega dangerous is if things like the Air traffic control systems get hacked into -- and I think even the Israeli's "Iron Dome" is being hacked by various people too. (Whether it should be or not is outside the scope of this post -- there's plenty of other places to air views on that particular dispute) - but as we become increasingly reliant on computers for more and more sophisticated things then we need to be very vigilant.
A lot of large cities for example are introducing driverless Metro / Subway trains -- if the system got hacked - all sorts of really nasty consequencies could occur. A lot of these systems not being regarded as "Strategic" and run by typical commercial organisations might not regard the possibility of tampering with the systems as even remotely possible. I'm sure a Metro accident on Paris Metro Line 1 (now being fully automated) just near ETOILE station at the height of the evening Rush hour would not be a pleasant thing with the authorities to deal with.