The Eustace Diamonds is pretty dark, as well. Though Lizzie is probably the most loathsome character in the book (and even then you often feel for her), she is surrounded by such flawed colleagues that you have trouble ever really liking any of them. The good ones are often stupidly loyal or weak or misguided. But it doesn't make the book any less enjoyable, because each character is so rich. It's also a fascinating exposure on marriage practices among the upper classes in 19th century Britain. Trollope's big theme, especially prevalent in this book, is the difficulty of being in the between classes: not rich enough to comfortably afford all the things you need to be in society but not poor enough to not have to care about them. All the principal characters suffer from this affliction and the remedy is marriage. The problem is that when everyone around you also wants to move up via marriage, it becomes a complex game of negotiations and investigations. Love is window-dressing at best in this world.
It was really interesting to read this book after having read The Gamekeeper. That book was about the warden in charge of maintaining the hunting grounds of a lord. In The Eustace Diamonds, we see a glimpse of that world but from the upper side. I don't think I would have fully appreciated the costs and difficulties in maintaining their land that was a part of the responsibilities of the landed gentry had I not read that book.