If you're going to kill everyone you don't need a system to discriminate individual targets, you just need to recognize any target broadly which you can do with even older technology.
Which is to say: there are significant downsides to such an approach to the extent that this is fit for a spooky story or an abysmal horror flick but not compatible with actual reality.
Cheap drones do change a lot about how wars are fought, sure. They are in that sense extremely relevant. But that does not mean that your scenario or similar scenarios like it are remotely realistic.
Put another way: Cheap drones are not nuclear weapons (which actually did lead to a earth shattering change in global power dynamics, though I would argue even their transformative hasn’t been absolute).
Cheap drones are more like machine guns, tanks or airplanes. “Mere” new technology. Obviously hugely important and impactful, not earth shattering.
(As for whether AI might be earth shattering, I do not know nor do I even want to get into that question here.)