I think the fear of AGI destroying humanity mostly stems from the suspicion that a logically thinking device would indeed come to the conclusion that we need to be exterminated.
If you make an AGI to run a paperclip factory, eventually you'll come back to find most of the carbon and iron in your galaxy refactored into paperclips. The extinction of any local flora or fauna there is just a byproduct of that drive. See [2] for a brief rundown.
[2]: https://selfawaresystems.com/wp-content/uploads/2008/01/ai_d...
Given the costs to run such an AGI, it seems unlikely one would be left unattended for long.
But why exterminate humans? Why not create nature reserves for us and other life, for bioconservation and research? That’s been a feature of human societies going back thousands of years.
and the less serious concerns we look at the bigger the chance (of course the severity is less, so calculating the impact is not trivial), but "non-runaway AGI used for antisocial purposes" is pretty likely.
Generally only for things we find cute or pretty.
Plenty of "save the whales!", not much "save this strain of this bacteria before it goes extinct!".
If we made something like ourselves burn more capable we’d be terrified and in trouble.