At first i agree to that he just tried to kill bodyguard for mission because he is cold and logical type and the reason as you said, but when he showed emotion in the scene, i changed my mind. Usually, he made dicision logically without emotion related or hide it completely.and i don't think Dowun is a Yandere type, just a cold and very logical one, he didn't want her to be taken by bodyguard because he can't let him change her mind, he must marry her and have children, with her royal blood, his clan will finally life in luxury, top of status, forever as the ruler, mission accomplished.