I watched this film right after I had read the book, and I didn't really have high expectations, but this was bad. I have two main criticisms.
1. The Writing
I was able to ignore and move past the multiple inconstancies with the book, because I could see they were taking it in a new direction. The direction was bad. I understand the metaphor they were going for, but they executed it so badly. They should have taken this concept and made a movie unrelated to Carmilla. Relating it to this book is what made it bad. If it was it's own unrelated thing, it would be fine, but this was not Carmilla.
2. The screenplay
Overall the acting wasn't bad, and the flow of the story was good, but the amount of time they spent exaggerating scenes is crazy. The bug scenes were cool, but I feel like there were too many. The dramatic scenes, especially the two at the end of Laura screaming, and her father carrying her, were insanely longer than they needed to be. There were multiple transition scenes that were just way longer than they needed to be. The movie is about an hour and a half long, and I bet if you cut the scenes to normal length, you'd lose at least ten minutes.