The novelist Richard Russo in a fine, engaging commentary about Elliot Spitzer and how his story has drearily been oversimplified by the press, notes:
unrelenting virtue is not just unrealistic but uninteresting.
He’s not the first one to have that thought. It was a central theme of Robert Louis Stevenson’s works. Hyde is more interesting than the virtuous Jekyll. Must Jekyll become Hyde because he’s addicted to the drug or because evil, Hyde, is more interesting? The character that everyone remembers from Treasure Island isn’t the good Squire Trelawney but the treacherous, frankly evil Long John Silver. Adam Breck is more interesting than Mr. Rankeillor.
The theme is expressed most graphically in The Master of Ballantrae. It is the wicked brother, the Master, the older son, James, who is interesting. The younger son, Henry, is a stick.
In Paradise Lost the most intriguing figure isn’t the doltish Adam or the priggish God but Lucifer.
However, I can’t say I agree with the notion that evil is more interesting than good. There are wonderfully fascinating fictional characters who are thoroughly good but they are imperfect. Each is damaged, flawed, in some way. Quixote is mad. Prince Myshkin has epilepsy. Superman must have his kryptonite.