I think to a certain extent Duchess is here to kickstart the plot: a lot of the book could be described as "Duchess does a thing and everything else scrambles to rearrange the plans his actions have shattered." This is a style of plotting that I find stressful (I almost always dislike wedding episodes in TV shows because they *always* seem to involve the wedding going pear-shaped in this manner) but other people seem to find it entertaining.

I don't think we're meant to think that Duchess is born bad: he's clearly been shaped by a rough childhood with a neglectful conman of a father. I think the argument rather is that just because someone has some good qualities (Duchess clearly feels a genuine desire to make other people happy, for instance), and sympathetic reasons for his bad qualities, that doesn't mean you HAVE to stick by him even after he has amply demonstrated that he's just going to keep burning down your life.