I think that whenever they release the sequel to BG2 they'd have to implement more options as to whether you want to be good or evil and have an actual storyline for the evil char. Something which is not in BG2.
Just a thought though...

I'm pretty sure BG2 would have ended the same way regardless of your alignment. The idea was just to get your soul back, yes?

You ARE supposed to be a hero remember. That's the way the plot is written. THe expansion and Baldurs gate three will assume you're a hero. You'd need two different games if you wanted more evil plots.