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.