@Liama Which enemy she kept alive? The old lady? She didn't care too much about her so when Bek Han asked with the purest of intentions, and she was like "whatever, sure". If the old lady would actually do something that will bite them in the ass (aside from hiding the runaway princess), then it would reflect quite poorly on Baek Han, but so far she didn't had a direct negative impact.
And need I remind you that it was Dowun father that kept the other princess alive, to which Dowun eventually agreed? It seems they kept the enemy alive, possibly to use her for their own agenda. Not sure if he sees it coming, but his father WILL backstab him. Don't forget that his brother's inevitable treason will also be his fault. And he keeps misreading Lyon because he has no idea how to treat her.
I think Dowun has the longer record when it comes to making mistakes....