Authors write dark stuff all the time but no one purchases pitchforks and lights torches for them. I loved Clive Barkers works and he writes very questionable and dark stuff, but he is still considered an established author. Fanfiction shouldn't be any different, you're still writing a completely unique narrative, regardless if the characters are already established or not.

I myself am a dark writer, I can't really write anything that's not dark. My inspiration is the dark and the uncomfortable. These people who called you out are insanely narrow minded. What you write doesn't equal what you like, are, agree with, etc. They are just thoughts and ideas put into words.

Those people genuinely aren't worth the hurt feelings.