I like grimdarks, but not every grimdark. I think that, grimdarks (no matter- art or fanfic or music) must have sense, like every kind of art. Unfortunately, many artists who make grimdarks completely forget about it. Slaughter only for slaughter, violence only for violence, murders without story or even reason- it is completely unthinkable for me. It's just stupid, nothing also. Also, many of grimdark fanfics are similar. I know, maybe it's hard to write something original in story, witch is full of violence, but solution is simply- you have no idea, you don't write.
However, some grimdark arts/fanfics with amazing story, credible reason for events (it's mean violence or murders of course) and I very, very like it.