1. Computer is smart.
2. Computer realises people are doing the wrong thing and that, for the good of the world ("I, Robot"), or the country ("Eagle Eye"), some people need to die.
3. Man disagrees with an evidently vastly superior intellect and shuts down the computer because he has to be able to continue on his self destructive path.
There are variations on this, but it always boils down to: computers are bad and will get the better of us ("Fail-safe", "Terminator", "2001 - A Space Odyssey", "War Games", and a bunch more I can't recall), but we must prevail so we can continue on our path to destruction unaided.
The computer is right! In Eagle Eye, the reasoning was sound: the top 10 US officials should have been eliminated (although the means were a little Heath Robinson-esque) in the interests of national security.
As for the means of which I speak: let's look at what a computer can and can't do.
1. A computer cannot cause a mechanical failure in a high tension power cable such that the cable falls down and electrocutes a guy, especially as, with no CCTV, the computer can't see the guy. (And why does he run along the line of the cables, rather than sideways?)
2. A computer cannot kill a guy with a robotic arm designed to move hard discs from A to B. Why would anyone equip such an arm with a servo-motor that strong?
3. A military computer is unlikely to be linked up to every network in the country and so the all-pervasive access to electronic devices just won't happen in the first place.
4. Why get the guy to travel all the way across the country to the command centre to key in his voice print to unlock the go-codes on the evil mission? Why not just phone up and say "Steve, (or whatever his name is"), can you say the following...?"), and then feed that recording into the security system to unlock the codes? For that matter, why can't the oh-so-smart computer just fake the voice print?
5. I don't care if the guy is an identical twin. His biometric data will be different to his twin.
6. Smuggling the exploding crystal into the House of Reps on a necklace would surely be easier if the necklace was passed off as a gift to any old female punter, rather than engineering the whole elaborate thing of getting the chick across the country, too.
Oh, and 7: Changing traffic lights. We saw this in The Italian Job, too. People don't respond instantly to traffic light changes. Suddenly changing them red without the orange won't cause the flow of traffic to instantly change, and people won't blindly follow the lights and drive out into cross traffic because the light says Go.
But as I say, entertaining. 3MT.