Being one of the people who didn't like this and who thought the ending felt somewhat out of place I'll try to explain why. Also considering the effect I seem to have on some people I'll need to emphasize that
these are my opinions and others are free to have their own. These are based on one viewing and it's entirely possible that I've missed something.
1) Early parts (and by early I mean something like 90 minutes) were extremely dull. Charlie's death (around 40 minutes if I remember correctly) was the first actual thing happening in the whole film. Before that the only relevant thing seemed to be the mother's speech in the grief counseling group.
After that the film
tried to focus on the aftermath of Charlie's death but all we got was couple of outbursts from the characters (it didn't help that I disliked Alex Wolff's performance) and repetition of earlier described sleep walk incident. Also it's hard for me to engage in the grief of fictional characters unless they're at least somewhat likable.
2) The spiritualism twist was horrible. I assume that it (and the Ouija board) are somehow bigger part of US "horror" culture but I've never liked it at all. It felt like a cheap plot device.
3) Like @
Sedai I thought it really weird that there was no legal action taken against Peter. He was under the influence of drugs while driving a car and someone died as a result. It's a wrong kind of leap of faith for a film like this.
4) About Paimon. First of all why was he possessing Charlie in the first place? Peter was male and much older so why did the granny choose to use the girl as a vessel when she knew Paimon preferred male bodies?
5) About Paimon II. This is more subjective than previous but was Paimon supposed to be little slow? Charlie acted all weird and felt really slow. Also in the end Peter just stared with empty eyes and had this feeble minded grin. I'd personally expect demon kings to have above average IQ
6) About Paimon III. This didn't actually affect my opinion of the film but I'm just curious. Why did Paimon (I suppose) make Peter break his nose in school?
7) Then the ending of the film (and by ending I mean the last 20 minutes or so starting from the death of the father). First obvious question is why did the father burn with the book? I'm not entirely sure why the mother caught fire earlier but the effect suddenly changing to father doesn't seem to make any sense.
8) Ending II. So was the mother possessed by Paimon or why did she go after Peter? If she really was possessed by Paimon then why did he choose her, again, over Peter who was also present during the seance and viable target after mother's death?
9) Ending III. I found it really weird that, in the middle of a scene, the mother suddenly gains the ability to levitate. Also what's with Paimon's fixation to decapitations or, in other words, why go to such trouble in killing the mom?
10) Ending IV. Mother's headless corpse flying to wood house just looked silly. And why did Paimon himself walk and climb the ladder when even the corpses of his worshipers flew?
11) Almost forgot. Why bring granny's corpse back to house?
Some of the above is nitpicking but I decided to write down everything I remembered thinking while watching the film. Some of these are really subjective and I won't be arguing for them too much.