Interpretations? *spoilers*

So after watching this movie I have my own theory about what the hell was actually going on. It's likely not spot-on, but just how I interpreted the events :)

I think there were no actual demons in this movie, for one. Everyone that appeared "demonic" I think was in fact either incredibly terrified/paranoid/experiencing some other heightened horrible emotion (the father that performed the ritual was in serious grief of his dead wife, for example) and was twisted in form and behaviour accordingly. Everything humans experience is subjective (meaning we all perceive things differently according to our own prejudices/fears/imaginations/level of sanity) and what may have simply been someone's face twisted with terror (such as the children when the father came after them) could have been perceived by him as "demonic".

I think what ultimately undid the group was the abundance of greed, paranoia, and regret experienced to some degree by most (not all, but most) of the characters. Those emotions coupled with the unsettling place they were staying and the initial meeting with what actually WAS a skinned boar at the beginning but they perceived to be something more so (and we as the audience are taking the trip with them, so we see demons too) simply gradually drove them all insane.

Of course, there's a lot of stuff that doesn't support this theory (such as the soldiers at the end perceiving one of the characters as a demon as well--though you could argue it was the cycle repeating itself in the form of war-torn human beings) but still makes you think a bit :)