If you think that machines can somehow possess the same qualities as a human brain, then why is it improbable to think that as with humans if it can attain controlling power that it won't?
That's assuming it will be a singular consciousness alone. Plus, even in the worse case situation such a consciousness will not be all powerful. Smash a few turbines or transformers in key locations across the world and you can black out 2/3's of the world's power grids. It doesn't take lasers or plasma rifles to fuck up a turbine either.
And you haven't given me a real motive as to why such a consciousness would need to attack or lash out. At worse, I believe it will act more like a teenager than a mass murderer (Honestly, I can't say which is worse sometimes...).