What is the theme in The Silence of the Lambs by Thomas Harris?

Desire is a theme in the book. The theme of desire in the book is really a theme of human motivation. Humans are motivated by desire, and the strength of the desire often determines the results of the actions it motivates. The various characters have various desires, and Dr. Hannibal Lecter's character provides a voice for this theme. During one of their notable conversations about Buffalo Bill, Lecter tries to get Clarice to look at Bill's motivation. What is the desire that drives him?