An AI doesn't need to kill even nearly 100% of humanity in order to render us a non-threat.

Also there are plenty of doomer scenarios along the lines of e.g. some ethnosupremacist cult making a virus that kills everyone outside of their group.

One of the scariest science fiction stories I've ever read is "The White Plague" by Frank Herbert... yeah, the Dune guy. In this story a molecular biologist's family is murdered in a terrorist event, goes insane, and decides to create a plague that is carried by men but kills women, so that everyone else has to experience the loss he did. It's not scary because the story is any more particularly filled with death or destruction or gore or other viscerally scary things like that. It was scary because as I was reading it, there was no hole in the logic I could take refuge in. There's simply no reason that book can't happen.

And that was my reaction 15 or 20 years ago. It is only harder to poke holes in the logic now than it was then.