i have a few thoughts to comment on what's been said, hopefully not too controversial
A self governing system does not need to be intelligent to be dangerous. i think this is what scares people most. We more and more give automated systems the power to do more advanced and crucial tasks I think eventually we would reach a point where it might be "safer" to give the choice to an automated machine than to a person. mind you this machine can be something we already have today.
i don't think an AI that can compete with human behaviour can explode instantly out of a single creation. i think we're more likely to experience advances upon advances in the field towards forming bits and pieces of the human mind.
i find it very unrealistic to think that a machine will simply come to life, i think this stems out of our belief in a soul or a spark of life given by a creator. i think like most machines it will evolve gradually until it reaches a point where it is relevant. i don't think anyone will even notice the change.
Also, there this unfounded image of how an AI would be; rational, not prone to impulses temptations, poetically machine-like, and non-human like. the way we saw machines years and years ago. (that's movies for ya)
Creating something that can learn from others will require it to empathise with others, i think it's only science fiction that an AI could be created with the full knowledge of it's operations. artificial intelligence is by essence heuristic, it would learn and adapt to it's surroundings.
I think it would be a very unintelligent machine for it to try to kill off any means it has to survive as an intelligence. society is the root of intelligence. communication, language etc..
My views maybe a bit optimistic around the subject. but i never hear them spoken out loud.