I must say, I'm glad I watched the series Vikings before this, as it prepared me for a lot of what I saw. I would call this an update of the original Wickerman, but gross.
Some have complained that we didn't need the opening section with Dani's family for the movie to work, but I disagree. We needed it to (1) understand the level of pain she is dealing with, (2) understand why she would be so willing to turn on Christian in the end, and (3) empathize with her at the outset. Without the opening section, she can more easily be dismissed as the "moody, kooky girlfriend." Instead we're right there with her and understand the enormity of what she's been dealing with, which in turn makes us that much less sympathetic toward Christian.
ETA: Consequently, I wonder what happened to that friend she spoke to on the phone in the beginning. I kept thinking: "Listen to your friend; she is so spot on." Then poof! we never heard from her again.