I suppose the novel doesn't leave as many loose ends as this film. Or it felt that way to me, the characters' individual stories are not developed, you can't really understand why they are that way...so some details come across as unnecessary since they end up not adding anything to the story.
Didn't want to mention spoilers but the first time Josefin appears, it's very obvious she's dead. And didn't get the last scene, where Alena flees the party: her girlfriend goes after her one second later but for some reason Alena manages to fight and kill two people before she's reached... Entertaining, maybe, but very forgettable, I'd say.
Review of Alena
