Well this can be done with a couple of variables.
Have a variable called Evil meter.(Example).
Make some events where you can increase the option of being good or bad, for example, a woman would say help me!
You have a choice to say yes or no.
Yes would make the woman say yay thanks!
No would say "yur evil!" and make a variable operation adding 1 point.
Then when you make normal events, like fable, and they should act differently, then make a normal page 1 with the message they say, then make as many pages as you want depending on how evil you are using the variable to choose the way the speak to you.
For the ultimate affect, make it so the eviler you get, your class changes and extra skills can be learned !