Plot hole?

I just realized: Sun's brother sent in those fake prison guards to kill her, right? But if he could do that, couldn't they just help free her instead? I know this is after he kills their dad and Sun swears revenge on him, but why didn't he think to do it before? Like "Dad wait, I think I know a way we can save Sun, save face, and I don't kill you" I know it's a bit of a nit pick, but it still felt like it was worth pointing out.