[Houshalter]

Once we get smarter than human AI, it can improve itself or make even better AIs, which make better AIs, and so on. It could very rapidly become far, far more intelligent than us. A being that powerful could do whatever it wants. Manipulate humans, hack the entire internet, design nanotechnology, etc. It's very unlikely that it would have human morality, and so wouldn't care about eliminating us to turn the Earth into a giant supercomputer, or just prevent us from ever creating competing AIs.