Chemist here, what bioweapon do you know of that can wipe out 8 billion people before anyone can do something about it? Never mind one release by a single person.
It's really not 'trivial' to slaughter millions of people.
Such a bioweapon (probably) doesn't exist yet, but that doesn't mean it's impossible in principle. It's "trivial" in the sense that it's easy to _imagine_ such a thing... but a superintelligence is by definition much much smarter than humans, so whatever it comes up with will likely be very surprising to us.
Or... something so far outside of the box that we'd have about as much understanding of it as a baboon does a gun.