I thought that the mistake was made by the compulsion laden messenger himself - not by Graendal. I was under the impression that he was simply following a long-standing order to clean up loose ends. In addition, I also gathered that the compulsion guy didn't really know what he was doing, hence the slow-acting poison. He was just following orders with what he knew. I'm not sure if this is actually true - my memory of this passage is pretty blurry. I'll have to go back and re-read it to make sure my facts are straight ...