for something to happen and suspense to be built (let's face it, stupid decisions are the bread and butter of just any movie). But why the *beep* did they consciously decide to stay when they discovered the wicked demon possession. They didn't even try to escape first. The mother says to the older daughter "Leave" with the intention the mother and priest will stay. Older daughter refuses, saying it's her sister and "splitting up is a bad idea"
So splitting up is bad...but staying to fight a demon that even a priest is ill-equiped to deal with is a great idea??? No, no you can't lampshade a horror trope when you're making just bad of a decision. And all they needed to do was try to escape BEFORE they decided to fight this demon. The one change and I could forgive that stupidity.
She was right, splitting up is a stupid decision 9 out of 10 times, however the not wanting to leave family was normal. What should she have said? "Screw it, she's been my daughter for ten years now, she had a good run. You hear that demons? YOU CAN KEEP HER!" They didn't know that she was beyond saving and they had a damn priest there who seemed to know what he was talking about.
Don't get me wrong, I don't think it was a great film and the whole ending (from maybe just before the hanging?) was pretty terrible, but I don't think it's that easy to abandon a family member. Also you say they should have escaped before fighting the demon, how can they fight it from outside? And who knows if they daughter will still be alive by the time they come back with an army of priests.
By which point the daughter could be dead, or could have run away etc. Again, I think you overestimate how easy it is to leave a family member to chill with demons/ghosts. Also again, the priest acted like he knew what he was doing. Who are they to say otherwise, they don't know anything about how it works.
But the priest clearly states they were dealing with "other forces". He didn't know how powerful the demon was and plus you have to have proof of a possession in order to get an exorcism from the church you just can't "do it". By the time they realized how powerful it was they were already locked in the house.
That's funny. I can understand the family sticking together more than if they just left the little girl, but the smart thing would have been to at least have someone leave and get help.
What bothered me more, was the priest going upstairs to talk about the spirit and how he tricked it, and then go on about the whole situation IN THE HOUSE where the possessed child and spirit known to have been there all along are there. DUH! Like it isn't going to know at that point it's outed? He should at least know the basics of how spirit works, or nothing else have common sense. Even if he only thought it was reading minds, could it not still read minds from down stairs? Really dumb.
That was the thing that bugged me the most too. How were they not getting the irony of "this demon knows everything we've ever said inside the house", while still in the house! The priest seemed to be very smart except for that one pretty major slip-up.