You don’t need to attack humans directly. Simply making and distributing a virus or fungus that successfully damages the grasses that we use for the bulk of our calories would do the job.
You'd need to hit wheat, corn AND rice at the same time. And we probably have 10 others we're not using because those 3 are already enough. It would be a tragedy but we can eat lots of things.
They are very closely related and share a number of fungal pathogens. In addition as grown they are almost a small set of monocultures.
"Simply"? You might as well say the same thing about creating a doomsday virus.
Yes, with AI I will be able to simply build a doomsday machine. Claude will just tell me how to build a supply chain, circumvent regulations and do this without anyone finding out.