Just curious, but why does it almost always have to Edward who's "dark"? I know he's the man, well, almost-sort of, but still. Does he always have to be the one to do the "dark" things?
I'm not really a Darkward person myself, nor do I read a lot of stories where Edward is some sort of mobster who becomes somehow conflicted in his life the moment he met Bella, when before he had no problem with it whatsoever.
Now, when I read Twilight and in turn, Midnight Sun, I felt like there were two sides of him constantly dueling for power. There was the overbearing, overprotective, manipulative, verbally abusive ass he was with Bella, and then there was Edward the way he was with his family. Bella, to me, brought out his bad side -- the side that told him she was wrong and he was right. Maybe that's where it comes from.
But nonetheless, a "dark" story can have lots of elements that shouldn't stem just from Edward but from Bella as well. I guess it comes down to what you thought when you read the books as to what you can handle in FanFiction.
I've read first chapters of quite a few stories, and even some further in, and I don't really like extremely domineering, sailor-mouthed, majorly abusive Edward who's like this for reasons unknown until you get further into the story. I don't want to read stories like that when they warp nearly everything in books around to fit the Edward they want to write about.
As far as rape in stories, I'm a big girl, and I can handle a lot. As long as it's not overly described and isn't there as a means to an end, I'll read it and probably move on, no harm done. That being said, I wrote a rape scene in a new story I just started, and I guess I'll have to wait and see what people think of it if I decide to post it.