if you wanted to represent it through a game mechanic, this would probably be the best way - like becoming the Slayer in BG2 = a good way to beat tough fights but you start turning evil.
what i dislike about that approach is that it makes the slider a guage of roleplaying,
it'd be better to implement it through the story itself. problem is, it's hard to create visceral emotions in a CRPG setting, such that you would forgo what you know to be the more sensible course of action because you're overcome by anger, fear, hatred, etc.