on Writers Helping Writers:

Every story needs a great villain! Sometimes, that villain is obvious–someone who’s not a part of your protagonist’s gang of supporters against whom everyone is fighting. Think Sauron in LORD OF THE RINGS or the White Witch in the CHRONICLES OF NARNIA.
But what if your villain ends up being someone right under the reader’s nose? This will be true if you’re writing a whodunnit or any story with betrayal on a grand scale. How do we keep this sort of villain hidden in plain view? Here are five ways to do just that: