Assuming that an MC who cared about his aunt is canonical, then killing He Gan is one of the few "must do's" that every MC, regardless of route, has. And this is probably one of the easiest. I mean, if the MC can't accomplish anything significant since the very first book, that just means he hasn't evolved at all.
Look, the author does what he wants, that's obvious. But something that defines almost all Wuxia/Xianxia protagonists, regardless of their personalities, is that they do what they have to do when the opportunity arises.