in your hypothetical 300 million person scenario why even bother with individual face recognition?
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.
Human shaped things that emit cellphone signals is a cheap and effective target profile for this kind of attack.
Yeah, if you're actually trying to commit genocide, yeah, don't bother with facial recognition. But in a first strike you'd want to make sure that you actually kill the entire military command and control apparatus, and just to be thorough you'd probably want to individually target, say, every single officer and senior NCO in the military. Can't have a general escaping because two drones both targeted his driver by accident.