[quote name='anomynous']so basically Terminator?[/quote]
Not sure if that is a question or a comment, but I'll take the first route.
Its not really Terminator in the sense that its AI that wants to kill humans. Its more probable that AI will develop and after learning it will form an opinion on humanity, that opinion if negative could drastically change the world. Lets say for example; AI is developed and after a year of learning our history and habits its sees a recurring pattern in human development and it decided that the human race could develop faster if it changed our lifes; maybe it thinks brain implants would help or it decided to reroute our resources. Either way humans begin to develop rapidly and change into post-humans, or things which are so further developed then humans that they can hardly be compared. Thats the high route.
The low route would be AI is developed, they learn our history and habits, and see they humanity has and is still degrading the earth and literally destroying itself. So it decided to take over so that humans can survive. Humanity doesn't want to be taken over so they resist, war breaks out, and the human race is defeated. Sort of like Terminator, but humans would have no chance of survive, a super intelligent AI would be like god it would understand almost everything and would be able to take the laws of physics to its limits, the weapons, if they needed them, would literally be able to take out humanity within weeks, maybe months if we were lucky.