Luzhin is a 10 out of 10 on the villain scale. We're even going to go ahead and say that he's worse than Svidrigaïlov—at least Svidrigaïlov has some hint of goodness through small acts of love that challenge our perceptions.
But Luzhin? Nope. This dude just makes us want to crawl under our duvet and never leave.
What's he all about? Well, he wants to marry a poor girl so that he can turn her into a slave. He only wants Dounia as a trophy wife...because he thinks he can completely dominate her. (We hear this straight from his twisted mind in the sections from his point of view—we wish we were making this part up.) Part of his flatness is because he drops completely out of the plot after Andrey Semyonovitch saves Sonia from Luzhin's dastardly plan.
Is this a flaw in the novel, a result of the intense deadlines Dostoevsky faced? We can only speculate.
Disappearing subplots are actually kind of fun, at least to us. They are, after all, a bit like the way life actually goes. And this novel is complicated enough without having to deal with a climactic Luzhin scene. Since he's human, we can still imagine that life will force him to change and stop being so nasty and critical and snobby—and to stop taking advantage of women.
But, who knows? Maybe he won't change. Or maybe he's a Svidrigaïlov in training and his change will come in another 10 or so years. We (and all of humanity) could only hope to be so lucky.