I suppose the most true-to-myth version would be to allow a player to change alignment at critical junctures, and to have NPCs react to them based on the length of time they have spent acting in accordance with their alignments, with a close-to-peak bonus for time approximating one segment of the overall story.

Erm. I mean, if you start out evil, you can change to neutral by sparing the kidnapped heroine, or to good by freeing her. And a Good NPC will be wary of you immediately after the change, but accord you full respect if you continue to act Good down the road.

Gosh darn it, there ought to be a better way to model treachery.