The advisor is depicted as a smart and careful guy, so it would be quite strange for him to try to poison Yona right now, while the army is celebrating the victory, with Hak having played a big role to guide them to that victory. It's not like the advisor would have hunted her for personal reasons, it was merely to make sure Soo-Won's position as a king wouldn't be challenged by a stronger opponent, leading the country into being split in two. However, if he murdered her right now, the army and the whole Fire Tribe likely would be extremely displeased, which wouldn't help Soo-Won.