I am going to say scientific experiment gone wrong. Pocket black hole, universal solvent, super bomb actually working when tested, whatever. Someone with a white coat will do something that will go uh-uh and we may not have time to watch it in the news before we all die.
How about an asteroid impact that liquifies the Earth ? Or a nearby supernova ? Or a passing rogue planet yanks Earth out of orbit and we freeze or burn ?
If we guess correctly, & win, how do we collect the bet?
Just grab it. I promise there will be no contention.
I’m pushing for Tommy Westphall
More seriously? While I don’t know that it would destroy all human life, society in general is going to go down by escalating wars that develop in response to limited resources, along with environmental changes that make these resources even scarcer.
Perhaps, but one must keep in mind that for destruction ice is also great and would suffice.