Chemical and biological warfare is hard. A cult in Japan created a mass casualty event using nerve gas. Which is the only somewhat "successful" terrorist WMD attack I know of. Knowledge of how to create these weapons isn't new. Guns and bombs are the most widely used terror weapons for a reason.
Enter LLMs to help through all those pesky hard parts.
The hard part is probably finding the lab equipment and chemicals without being noticed, and choosing not to use it to manufacture drugs instead which would be much more profitable