It's not a terrible movie.
The words "based on" or "adapted from" actually mean "based on" and "adapted from", not the exact, story. The movie could be viewed as a fan fiction or like watching a drama that was remade in different countries. You may like one version and you may not like another.
I find it odd that fans who are unhappy with this movie simply dont just blame your favorite writer for selling out their work to a production company that have a different creative vision.