A.I.'s aren't all that bad if they are programmed to have breakwalls where their programming logic does not go past the breakwall.
The real horror is having a robot built with the intelligence of the HeartBleed Virus. Now that would be scary.
AI's are always built in with safeguards lest they turn against their creators (at least in fiction) and they always manage to circumvent or deactivate these at some point.
My knowledge of AI programming is nearly nonexistant so i don't know how that could be even done. If an AI comes into existence it has the potential to outclass a human brain by far, given enough hardware it may outclass all of humankind's brainpower.
Can we honestly believe that it then wouldn't find a way to transcend its original programming and develop goals and wishes of its own?
This is where the fear and concern comes from.. something that powerful could easily either wipe humankind out or at least bring it near extinction when it perceives us as threats to its existence or goals.
Or it may follow the route of the AI depicted in "Her" (awesome movie btw, should be seen).