He's cursed even though he didn't do anything... Of course he would want to lift it. And if his son marrying a half spirit who happens to be of royal lineage is the answer, then of course he'd try to arrange it. People get married because of less, look at the hero's brother who had to get married to his brother's fiancée. That doesn't make him evil. He's doing what's best for his family.

it seems that the prince want the girl since not only she's a spirit she's also gifted making the linage of the royal family better if she marries one of the royal family their curse may be lifted