There are some legitimate concerns about AI that sound almost like something out of the Book of Revelations, but are entirely rational.
The biggest one is the idea of AI being used to implement "automated con artistry at scale." Imagine assigning every living human being a virtual AI powered con artist to tail them around and try to convince them of whatever is ordered by the highest bidder. This AI is powered by mass surveillance and big data and is able to know the "mark" intimately and work on them 24/7 both directly and via their social network. Now throw in deep fakes, attention maximizing "compulsion loops" and other adversarial models of human cognition, etc.
It's basically the hydrogen bomb of propaganda, a doomsday machine for the mind. It would be like creating Satan, except this one would be entirely amoral and mercenary.
At the very least this would be the end of democracy in any form. I could see this ushering in the permanent eternal victory of totalitarianism since I can't imagine the masses being able to organize any resistance when constantly bombarded with propaganda from their demons. The new feudal aristocracy would be the ones running the demons.
This is one of the darkest plausible visions of the future I can think of at the moment, far worse than anything related to climate change or similar problems. If we manage to avoid these kinds of scenarios but still drown Miami I'll say we didn't do too badly.
Transhumanism comes with some analogous concerns around the potential for enhanced humans to enslave the rest of humanity through superior cognitive capacities and the incredible accumulation of wealth. I can imagine a scenario where a few wealthy people gain access to technologies for life extension and cognitive enhancement and then "run away," effectively becoming a new species and exterminating or enslaving "legacy humans." This isn't mutually exclusive to the AI hellscape I describe above, since that would be an ideal mechanism to enslave the rest.
Ultimately I think these technologies are all neutral. The problem is that we are not. I don't fear AI. I fear what humans will do with AI.