One bad possibility is that AI & robotics advance to the point where they can do every job better and more cheaply than humans; and then humans are no longer employable and all die if they have insufficient capital to survive the period between unemployment and post-scarcity.

Another possibility is that, once AI exceeds human performance in all economically useful activities, including high-level planning, governance, law enforcement, and military actions, it discovers that the benefits of keeping humans around aren't worth the costs and risks.