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.
