Sydney Bristow may be the star of Alias, but for some of us the most compelling character is her nemesis Arvin Sloane: a cold, ruthless betrayer who commits some of the show’s most vile acts but remains tormented by a core of essential humanity.

Why is he so fascinating?

Because he’s the very best kind of villain.

Stories that require villains, like (picking an example at random) fast-paced spy thrillers about pretty ladies who get sent on death-defying missions, only work to the extent that the villains make some kind of narrative sense. Few writers are talented enough to get away with villains who just skulk about, being evil and doing bad things, because they have some kind of deep, inborn personal love of sneering. Shakespeare did with Iago, whose malice toward Othello bears no explanation beyond, possibly, getting out of the wrong side of the bed every morning of his life; but come on, this was Shakespeare, and his refusal to provide an explanation has provided generations of academics with a form of full-time employment that consists entirely of arguing with each other about the precise species of bug Iago stored six inches up his intestinal tract.

In practice, storytellers in need of villains need to pick theirs from a very short list of basic models, which we’re about to rank in order of increasing complexity. Make no mistake: all can be done well. (Iago may belong to the least promising category, but he’ll be remembered the longest.) But  …