Q. Why is the Duke of Parford such a big jerk?
A. At base, the Duke of Parford has never wanted, and rarely needed, to care about anyone other than himself, and it shows.
A few things have aggravated his mean tendencies. He was in love (such as these things are) with his first wife and he was bitter about his parents shipping her off. He spent decades mourning her (and also being pissed off that he didn’t get to win). In the end, when he married a second time, he felt even more bitter about the woman he ended up marrying.
But he’s particularly a jerk—and almost childishly so in Unveiled—because of his medical history. Throughout the book, he’s having an ongoing series of transient ischemic attacks (sometimes called mini-strokes). Some of the things that he does—like when Margaret accuses him of making faces at the maids—are involuntary reactions on his part. He’s not making faces; he has temporarily lost control over his facial muscles. His refusal to fess up to this is entirely fear-based. He has little enough control over his life; he’s not about to admit that he’s not in control all the time.
In addition, the initial stroke that he had messed up his brain, resulting in some fairly severe personality changes. He’s petulant and childish because sometimes that happens to people who have strokes. We all have elements of our personality that are not very nice, but we’ve also learned to moderate those impulses and not go with our first, most selfish responses. The part of Parford’s brain that allows him to moderate exactly how much of a jerk he is has been damaged.