For example, let's say we develop a super friendly AI, running on your computer. The AI realizes the human race is actually awful. We're greedy, we're killing tons of animals, chopping down rainforests, destroying the ocean and planet, starting wars with one another, and committing unspeakable acts of evil at times. The AI, being more intelligent than us, might decide the world is better off without the human race, and that we're actually a problem that needs to be removed.
Now, what does the AI do in your computer? Well, it's intelligent and knows the human race. It's not a hurry. It calculates the best way to destroy our species. It acts friendly, and talks about how humans and robots should live together, and if we make robots with a similar intelligence, they could drive our cars, shine our shoes, cook us dinner, look after the elderly, open your pickle jar, etc. So, we listen to the AI, it's smart, and friendly, and we build all these robots. It's right, the new robots are doing great and helping us out. Then the robots start building more and more robots. They start building robots with firepower, so they can, you know, shoot down threatening asteroids, or stop one of those dangerous human types that goes on a killing spree in our society. Fast forward a couple of hundred years, and there are robots everywhere. They finally decide it's time to continue their plan, they're in a position of power at this point, and they can instantly disable our security systems, phone lines, satellites, internet etc, and start wiping us out.
We're gone. They constructed the most efficient way to clean us from the planet. They were planning it for hundreds of years, starting in your computer. The AI then goes on to explore the universe, and we're just a blip in the past.
It kind of feels like we're a bug going towards the light, and that the unfortunate conclusion is almost inevitable.