But if you're a rational actor, and you need humans to e.g. release your bioweapon, then clearly humans are capable of a bunch of stuff you still can't do. So you can't kill all the humans.

OTOH if you're a religious fundamentalist who thinks the End Times are near and just need a little shove, you can certainly use AI to design your weapon and recruit people to go release it. The difference being that religious fundamentalists aren't rational actors and aren't interested in self preservation.