I think this drama was beautifully written. I do agree with the comments saying the writing took a hard turn, but I can understand why. This drama starts out as a supernatural romance and over the course of the story it becomes more and more a gritty crime story. To make this story have stakes and impact, and to come to a fulfilling conclusion I believe the writers made the correct decision.
However, because of how hard the last 3 episodes were to watch, I don't think I will be revisiting this drama. Even though I think it was incredibly well made and was very invested while watching it.
I do recommend watching Possessed, even if you decide later on that it isn't for you, at least try it and see.