A lot of people just want more and more freedom to play characters that we haven't written, because, as you certainly must know, every decision, every character trait, needs to be written into the game in order for it to be possible for you, the player, to find it.
BioWare games are, above all, heroic tales of triumph over some great force or obstacle or issue. Yes, some of our characters walk the fine line between righteous and self-serving, but they all still have to want to overcome that force/issue/obstacle. Giving an "evil" character the ability to do some very bad things along the way kind of mucks up our story just a bit. There's a huge difference between a jerky hero having different motivations or attitudes about getting tot he end of the game, but it's another thing entirely for an evil character to start laying waste about him with sword and spell, then toddling off to run a used camel dealership in South Jersey.
You can't be "the bad guy." "The bad guy" is the guy you're usually trying to beat at the end of the game. At worst, you can be a really disagreeable hero or a good guy with questionable tastes or that jerk who saved the world. But playing "the bad guy" is not what you're going to find in a BioWare game. Sorry.
Also, in Dragon Age II, you don't save the world.