I also regularly think about such doomsday scenarios, and agree it's sadly likely that at some point (probably not in our lifetimes, but maybe it will be...), it will become practical for a single intelligent, resourceful individual to kill millions of people, or more. I made another comment about this a few months ago: https://news.ycombinator.com/item?id=21042022

I unfortunately think it'll likely successfully occur at least once, and I think there will be other attempts from copycats (like people who idolize the Columbine shooters), Aum Shinrikyo-style death cults, terrorist organizations, and perhaps unintentionally from nation-states who greatly underestimate the capability of the deadly technology they're using. Maybe even intentionally from nation-states, if their government becomes hijacked by a terrorist group or death cult. I think such people will poke at every possible dimension: nuclear/biological/chemical weapons, nano-bots like that Black Mirror killer bee episode, seeding some sort of thing into the atmosphere to destroy it... there are going to be some people who will genuinely dedicate their lives to trying to end all known intelligent life, and there is a horrifying chance some/one of them might fully succeed; perhaps a small group or a lone wolf.

It will be a vanishingly minuscule percentage of the population who would ever consider something so abhorrent, but of course it only takes one. If you traveled from the future back to today and handed an easy-to-use WMD to one of the mass shooters we keep hearing about on the news, a good percentage of them would probably deploy it with little hesitation. If the Las Vegas shooter invested so much effort to be able to massacre hundreds of strangers, and you handed him an easy way to massacre millions, he'd probably be overjoyed at all of the work you just saved him.

This is a horrifying image of humanity's future. I think this is probably a greater risk than some sort of WWIII, though that's definitely also a risk. Unfortunately, I see no reasonable solution to this other than most countries' citizens collectively accepting an all-encompassing Big Brother-style surveillance state. Even then, it still might all be for naught, since it obviously can't catch absolutely everything. But maybe it's the only way to give humanity a fighting chance (at least hypothetically until there's some way to achieve near-perfect immortality, even in response to a super-WMD, maybe by scattering the galaxy with quadrillions of pieces of technology that somehow allow most consciousnesses to retain their existences, or something crazy like that? who knows - such a thing probably couldn't exist even in 50,000 years).

"Those who would give up essential liberty, to purchase a little temporary safety, deserve neither liberty nor safety.", yes. But what if you must give up essential liberty in order to have any safety at all? I think this is a terrible and wrong argument in 2020, and probably also in 2080, and maybe also in 2120. But in 2320? 2620?

We also don't know how AI or perhaps AGI may play a role in all of this. This sort of absurdly asymmetric destruction may be possible long before AGI is created, or perhaps only long after it. AI or AGI could be a great aid both to the people trying to prevent this and the people trying to perpetrate it. Maybe an AI not that much more sophisticated than the ones we have today could have an impact.