She wasn't haunted. It kept pictures of her. It stared at her during her sleep. It called her name from time to time. She wasn't haunted. She was wanted. It was obsessed with her. Who knows why it wanted her, and who cares. What matters is that it wanted her, and it didn't want anyone else to have her. It was like a stalker, which is way more frightening than a haunting. After all, who cares why a guy is stalking a girl. What's scary is that we all know the lengths stalkers will go to have what they want.
I don't think the demon possessed her for the first time that last night. The demon had been possessing her throughout her life probably. After all, it possessed her a couple times during the movie. It obviously could do what it wanted with her. I think it was content to just have her to itself, but boyfriend provoked it so greatly that it finally took matters into its own hands. At that point, it's a logical step that the demon wouldn't leave her be. Once it completely took over her, I'm sure there was no turning back after that.
As for the childhood fire, why do you think it needed to possess her in order to catch her first house on fire? The demon displayed the ability to set fire to their home without anyone around when it caught the Jouji board on fire. I'd say, based on everything that was laid out in this movie, that it was probably trying to kill her family with the first fire. Like I said, the movie made it abundantly clear that this demon wanted her and no one else could have her.
I thought the ending was pretty frightening because I saw it coming halfway through the movie and kept hoping it wouldn't happen. It was the build up to the inevitability that made the ending scary for me.
Personally, I thought the only part of the movie that was goofy was that the demon didn't care if it was being recorded.