Assume Kerb was compelled beyond the ability for initiative and was truly an automaton.

Fact - Simple commands work best with compulsion. Refer to Verin's comments and the comments about Graendal's toys being useless for much beyond simple instructions because of the level of compulsion.

Assume only one prisoner under the shop. Given the discussion between Nynaeve and the warders, and that only Millisair is mentioned, it is likely the messenger was there alone as well.

Graedal decides to off the messenger. Her intel would have shown that Millisair uses that group of interrogators and that with the messenger going to her, she prepares a clean-up plan if necessary.

She activates the clean-up with a simple instruction to Kerb. Poison the prisoner. No name given because there is only one prisoner and it is safer if nobody knows the name. Also, with the messenger not talking, binding the instruction to the name might misfire if Kerb cannot connect the instruction to the prisoner.

Kerb poisons the prisoner.

Rand reacts like Rand does and sends Millisair to the same jail to suffer the same as the messenger did. She is now a prisoner, possibly in the same cell as the messenger was. Kerb's instructions are still there, so he poisons the prisoner.

There you have a clear, logical path with a simple instruction set that could result in an unintended poisoning of Millisair purely because Kerb was heavily compelled. Just like an industrial robot will happily weld clear air or the welding bench if no metal is present.