The basis for the Terminator movies is that this system, Skynet. In the movies, Skynet is built on the best hardware and software we humans can pile together (rumor has it the Pyramids were one of the initial attempts). The supersystem we created then proceeds to get rid of the problems - not even bothering with Asimov's laws - and a movie series was begun.
It's not a new story. It's a Frankenstein with asexual self-replication. Golems Gone Wild. Heinlein even played with it a bit in 'The Moon Is a Harsh Mistress'.
A predictable Skynet, though, is hardly a concern. Following brutal logic that lacks humanity (or maybe the wrong parts of it?), we can see how a super-intelligence would think that humans are problems that need to find... resolution.
An inefficient Skynet, on the other hand, would be horrid. Imagine, if you would, a system built in a hodge podge sort of way. It would 'break' now and then, creating unpredictable behaviors. Network congestion in this case would be nothing to sneeze at - the system might override itself when doing something predictable. It could just randomly kill people and then act innocent the rest of the time, completely unaware that it had - because, of course, it lacks logging ability. Or it might suddenly decide every human on the planet needs emails about erectile dysfunction and Russian wives.
It might take over your internet enabled device and control how brown your toast is, or get into a patient database at a hospital and decide to switch people's medications.
It might get bogged down while trying to figure out how to brew a proper cup of tea. (we miss you, DNA)
No, I'll take the rational Skynet any time.
Please, follow sensible software engineering practices.