If you really want to understand why thoughtful people are worried that AI may end up destroying humanity as we know it, you need to stop thinking of AI as having evil intent.
The key is to think of AIs as mindless survival machines. They have no sense of values, but they are very efficient problem solvers. If humanity gets in the way of their problem solving, they might get rid of humanity simply as a matter of efficiency.
The correct analogy is not the Borg from Star Trek or the Daleks from Doctor Who. The correct analogy is the broom from Fantasia.