I m so fkn confused. He needs someone to weild the sword, so he kills the knight that's already proven to be worthy of wielding the sword just to gamble on a child that may or may not be worthy in the future.
Not to mention he kills the parents despite having promised them that Mir can live with them until she turns 16 "because I can't wait that long!" But has to wait until she's 20 to marry her anyway.
Here's an idea, instead of making her hate you and then spend years trying to make her happy and grow up well, why not just LEAVE HER BE. None of this makes sense, literally none of it.
She would've been happy and grow up well with her family, and I'm pretty sure she'd still kill you if you asked her to. Fk man