You wouldnt make mustard gas. You'd make chlor gas,which is also very nasty but still quite a mile from mustard gas. The extent to which risky chemicals have been banned, reduced in concentration or made subject to extensive monitoring of sales and use is quite substantial.

But here is a huge difference to AI tools. Anyone could create these tools him or herself. It is information. Unlike information on how to build a nuke it is more easy to use this information for negative purposes, but the extend is much smaller. A deepfake itself cannot kill people. A selfmade pipebomb can. Meanwhile the cat is out of the hat for ML already. The tools are there and many people have copies of the code and it can be replicated countless times, whereas the clandestine bomb-builder needs to procure another batch of chemicals and hardware.

That's chlorine gas, you dunce.