What I really enjoy about FB is that anyone can write. It's like what the chef in Ratatouille kept saying. You have authors who are very skilled, and you have authors who are less so. While a skilled writer can flesh out a character and add subtlety, that's not the easiest thing to do. Sometimes if you want to get a plotline or idea out, you take shortcuts. Like others have mentioned, there are several branches that do give Sarah some respect, but they're less common because it's much easier just to make her a generic baddie, and if everyone avoided the easy road, we'd have a lot less writing on here, and we wouldn't be able to appreciate what those writers do bring to the table, which is often very good if you can get past the cliches.
Almost any branch that stays on Sarah for an appreciable period of time, which is to say, where the author devoted more time and effort than usual to Sarah, ends up with her at least somewhat humanized. Even, for example, the Paige McMillan story where Sarah is the clear antagonist the whole time, the opening post contains the following paragraph of Sarah's dialogue:
The rest of the branch stays true to that characterization.
I don't know if I'm allowed to toot my own horn here, but a while back I wrote a story from Sarah's perspective that paints her as very reasonable, and even as protagonist. It starts with The Guest's post here: http://fb.countd.eu/cgi-bin/fbstorypage.pl?page=091219233644The_Guest&path;=nospamstory
The branches where Sarah is reasonable exist if you search for them, but keep in mind that so long as Jon and Karyn are the protagonists, they're not going to like her, and neither is the reader. The question is simply how much subtlety the writer can introduce to Sarah's character, and so long as we keep FB open to authors of all skill levels (which I think is great), the lion's share of interpretations are going to lack that subtlety. It's just the nature of the beast.
But it's a good beast :)