It’s not a good novel without a great, complex, villain.
Not a, “Ha! I’m evil for evils sake!” but one, for example, like Irene Adler. She is a villain, she carries out crimes, tricks and manipulates the hero (Sherlock) and profits at the expense of others.
But if you go into context, for a woman of her time, that is a major accomplishment; to be on level (if not succeeding it) with a man.
I love the villain of the story.