Luzhin is a complete villain. Well, actually, an incomplete one, by dint of his completeness. That's a fancy way of saying he's a flat character. A complete villain, like Svidrigaïlov, has some hint of goodness, some small act of love that challenges our perceptions.
Luzhin doesn't. He really does want 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 mind in the sections for his point of view.) 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? (See "In A Nutshell" for more on that.) We can only speculate.
Disappearing subplots are actually kind of fun and amusing, at least to us. The 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 ten or so years. What do you think?