I don't really recommend the idea, because evil is something far too complicated to accommodate in multiple choice dialogue options. games today are only really capable of letting choose to be normal, or inexplicably psychotic.

if you try your hand at this though, what I would do is write a good story with good protagonist. don't worry initially about goodness, just write a really good story. then, with that done (at least a simple draft), rework the story in the way it would unfold if your protagonist was abhorantly selfish/greedy/power hungry. don't worry too much about the why . as has been said, people can fill in their own blanks.

what you need to establish first and foremost is an actual good present-tense (in other words - the current in-game events) story.

The problem of good and evil is an old story... I remember Ultima series and the famous "cards" chosen at the gypsy... I think that some questions at the beginning of the game plus some random elements are the best way to begin the game with some chosen path.

If you have a bastard soul, it's difficult you'll change during the game, except if you want to trick and anyway it would not be a role play anymore.