Can someone outline what an evil path looks like from a storytelling perspective?

For instance, I know what a good path looks like. You start, get a call to action of some sort, then face challenges along the way to overcoming an evil. There's a dark moment where it looks like you're not going to win, and at the climax you basically succeed or fail. That's kinda the simple version.

Now say you're evil. I guess you still get a call to action of some sort. You still have to face challenges along the way. But the challenges presented in the story are evil, by nature. So this becomes a story of evil overcoming evil? Or is your challenge to join the evil?

I mean, what exactly are people wanting to happen? What is it that you picture your character doing that your character can't do?

1. Joining Gortash? What does that look like? You just want two thrones at the end? One for you and one for Gortash?
2. Something else? What?

Maybe you could join Ketheric and lead the army to attack Baldur's Gate?

--I guess what I'm saying is that it feels like a completely new game would have to be written within the confines of what's happening. For example, if you join Gortash, what changes? Are you and Gortash suddenly hashing out plans, doing quests and overcoming some obstacle together?