Grimdark doesn't really strike me as an accurate description for Past Sins. It's certainly darker than the TV show. (I don't think Hasbro is likely to sign off on any gallows scenes anytime soon.) But I can't think of anything in the story that's worse than any of the classic Disney films.
Past Sins forced me to completely re-evaluate my hatred for fan-fic. It has its faults and I think it could be improved by a professional (and non-brony) editor taking a pass at it, but damn it's a great story.
personally i think the idea of past sins has some merit, but the way it was executed could use some revising. to all who are just making comments without at least saying a real reason for not liking it i say fooey to you. to those who are not just blurting out kill it with fire, i salute you. my personal opinion, i think making things from the fandom into canon ideas was better with derpy, but we all saw how that turned out.
Don't worry. Those haters are kinda asking for it, to be honest. It boggles me that they have to waste their energies spreading around that they don't like something and try to convince people not to like it instead of just leaving well enough alone and let those that actually do like it in peace.
Don't pay attention to the troll. If he doesn't like the character or the story that spawned her, he shouldn't was his time saying he doesn't like something. Besides, the worst thing in the fandom are close-minded punks like him that can't get it through their heads that just because they don't like something doesn't give them the right to go and post comments like his that amount to nothing but "I hate this thing that other people like and you should hate it to."