Don't read this as me saying that we are anywhere near it, or that LLMs are a stepping stone towards this scenario, but assuming inhumane hacking capabilities, it's not hard to think of how bots could change the way water treatment or energy plants operate, just enough to make large cities unsuitable for life. The line between drinkable water and not is thin, same goes for air quality, and mere days without electricity and you see some serious food supply chain issues.