You should not have evil and good be a simple and sudden choice. You should have one seemingly unimportant decision lead the player down a darker and darker path, where he thinks he is doing the right thing. If you are going to make an evil path, make it have motive. Do not have an option where you just suddenly start killing people, make it a long journey of betrayal where you need to kill someone for the greater good, and then another, and another. You keep killing, not for no reason, but because you think it will make the world better. If you are going to be good, you need to be flawed. There will come a time when you don't have food for the orphans, or when you must lie to save a friend. Make your character human, not an archetype. You want development, not a simple yes or no on whether you are a hero or a villain. Villains usually think they are heroes too.
