Castle: "What might drive a person to murder?"
Scott Powell: "I don't know. Revenge?"
Castle: "Revenge. A man wronged, his reputation besmirched. So he strikes out in passion. Some might consider his actions justified. What else might drive a person to kill?"
Scott Powell: "Being forced to read one of your books? [the class 'oohs'] Though that's probably more of a motive for suicide."