Also, an ISFP would have tertiary Ni -- enough to maybe appeal them to this 'dark side' that Dominant Ni comes up with, but not enough to join them. It's enough to give some connection, some proof that they may actually be a little alike, but not enough to draw the 'good' character to be 'evil.'
Just from the typelogic INTJ page alone:
Cassius (Julius Caesar)
Mr. Darcy (Pride and Prejudice)
Hannibal Lecter (Silence of the Lambs)
Professor Moriarty, Sherlock Holmes' nemesis
Rosencrantz and Guildenstern (Hamlet)
Hmm... most of these aren't even modern villains. Perhaps the root of this goes deeper than I first thought...