Could LLM's libel an individual? What's the duty of care for LLM's? Who would that person sue, should it come to that?
The last human finger that pressed the button, and anyone employing them to press the button. The real question is how much intent transfers. If you point a gun at a person and the trigger goes off without you pulling it, how liable are you? If you're pointing it at the ground and it does the same and the shrapnel flies about, how liable are you for that? If a loaded gun cooks off in a burning car and a bullet goes flying, how liable are you for that?
If you give an AI agent free reign to your computer and ask it to set your schedule, and it ends up sending classified weapons plans that happened to be on your computer to the Chinese Communist Party, how much should you be held culpable?
With novel technologies we typically answer this conservatively, and say that the person running the agent (or holding/owning the gun) has full civil liability for its actions regardless of their intent, but may limit criminal exposure.
We would (should) take an especially dire and suspicious view of anybody that has anything material to gain from using the tool irresponsibly or maliciously; We can demonstrate incentive/motive even if we cannot prove their intent. The law here is principally a deterrent against somebody that tells an agent "Win me this election" or "Build this product", and the agent then proceeds to hire a hitman to kill their opponent, or steal their rival's technology through industrial espionage. My fear is that the way things are going, it's a completely ineffective deterrent. My guess is a lot of people need to be killed by AI agents before we take it seriously and limit its use as a fig leaf.