But there is infinitely more atoms out in the Solar System and beyond... isn't a more likely scenario that the AI would quickly figure out how to blast out of Earth's gravity well and spread throughout the universe? It might accidentally kill a few humans in the process, but not intentionally...like a human crushing an ant when crossing the road.
That's assuming that the atoms around you aren't fighting back. Its simply easier for a machine to get itself into space to do whatever it needs to do rather than fight with the organic things on a planet.