It can only do what is possible. Stuff that's airgapped isn't getting hacked, so for instance you can safely exclude all nuclear related stuff. And building, deploying, and managing a virus would require a team of highly skilled humans - humans which could already carry out the apocalyptic possibilities by themselves if they so chose. We even have live samples of small pox still kicking about.

I just don't see much likely to happen beyond random websites getting hacked and hopefully companies (let alone countries) realizing that connecting critical systems to the internet is nothing short of stupid, even before LLMs. More generally, I expect LLMs are going to lead society to segue broadly away from the digital world, or at least beyond it. Not only because they're going to make a mess of everything digital, but because if they reach their potential then the digital domain, as far as typical problem solving goes, will basically be 'complete.' It's kind of like how the Industrial Revolution opened the door for society to move beyond agrarian economies. There's a vast amount of economic power being directed towards things LLMs should be able to 'solve' and, if so, then that's going to create an economic vacuum.