You paint a bleak future. Keep in mind though, there have been many dark moments in human history when a lot of people got killed for very bad reasons, and yet here we are.
is as easy and anonymous as buying porn on Amazon
I'm not sure ease of use is such a game changer. You can buy a drone today, completely anonymously, strap some explosives to it, remotely fly it into someone and detonate it, a few hundred yards away from you. Easily available cheap drones like that existed for at least a decade, yet I don't remember many cases where someone used them for this purpose. Does Slaughterbot-like product existence make it easier? If some terrorist wants to kill a bunch of people, how is it easier than just detonating a truck full of C4? To a terrorist this technology does not provide that much benefit over what's already available. How about governments? I don't see it - if a government wants someone dead, they will be dead (either officially, e.g. Bin Laden, or unofficially, Epstein-style). If a government wants a bunch of people dead, the difficulty lies not in technology, but in PR. I doubt there is a lack of trigger happy black ops types (or "patriots") ready to do whatever you can program a drone to do. Here I'm talking about democratic first world governments. It's even less clear if tyrannical governments would benefit a lot from this technology - sending a bunch of agents to arrest and execute people is just as effective. I don't think tactical difficulties of finding and physically shooting people is a big concern for decision makers. As you yourself pointed out, Khmer Rouge or North Korea had no problems doing that without any advanced technology.
you must kill them before they have a chance to deploy their forces.
Yes. And that's how it has been at least since 9/11 - CIA drone strikes all over the world. Honestly, I'd much rather have them only have done drone strikes if at all possible (instead of invading Iraq with boots on the ground).
more rapid and complete than nuclear weapons could ever have threatened
Sorry, I'm not seeing it - how would this change major conflicts and battlefields? If you have a battlefield, and you know who your enemy is, you don't really need Slaughterbots - you need big guns and missiles that can do real damage. It's much easier to defend soldiers against tiny drones than against heavy fire. If you don't know who your enemy is, say terrorists mixed in the crowd of civilians, how would face detection help you? As for precise military strikes - we're already doing it with drones, so nothing new here.
end up with everyone inevitably dead within a few days
enjoy what remains of our lives before it ends them
You are being overly dramatic. Yes, terrorists and evil governments will keep murdering people just like they always have. No, this technology does not make it fundamentally easier. Is the world today a scary place to live in? Yes, but for very different reasons - think about what will start happening in a few decades when the global temperature rises a couple degrees, triggering potentially cataclysmic events affecting livelihood of millions, or global pollution contaminating air, water and food to the point where it's making people sick. I really hope we will develop advanced technology by that time to deal with those issues.
But of course it's way more fun to discuss advanced defense methods against killer drones. So let's do that :) I was thinking that some kind of a small EMP device could have been used whenever slaughterbots are detected, but after reading a little about EMPs it seems it would not be able to hurt them much because these drones are so small. I don't think nets of any kind would be effective - I just don't see how would you cover a city with nets. Underground fortresses and off-planet camps can only protect a small number of people. In some scenarios some kind of laser based defense system could be effective (deployed in high value/risk environments), and of course we can keep tons of similar drones ready to attack other drones at multiple locations throughout the city. Neither of these seem to be particularly effective against a large scale attack, and both require very good mass surveillance. I think that a combination of very pervasive surveillance with an ability to deliver defense drones quickly to the area of the attack (perhaps carried in a missile, fired automatically as soon as a threat level calculated by the surveillance system crosses some threshold) is the best option. The defense drones could be much more expensive than the attack drones, so be able to quickly eliminate them. Fascinating engineering challenge!