Spoilers follow, although I doubt I can spoil the movie any more than the director already did. Although I doubt it makes a difference because my complaint has to do broadly with the plot of this film, my review is based off the US theatrical release.
It starts off as a fairly standard chainsaw massacre type of flick, which to be completely honest, has never been my favorite genre, and ends as something I didn't think existed: a slasher flick with a plot so awful that it loses any value it may have had as a simple gore fest. It turns out that the main character has multiple personalities and is also the killer, or something to that effect. It's very hard to say exactly what happened in the movie because after that revelation, it is obvious that 99% of the movie must be dismissed as nonsensical.
If this trite twist is done in a clever way, it can make piecing together the plot preceding the revelation an interesting endeavor, but this movie didn't bother with being clever. The main character would have to be in two different places at the same time in the film so many times that there is no point in trying to put it all together. There is a car chase with vehicles driven by different aspects of the personality; one aspect rides to a remote house with her friend, the other drives a souped up murder wagon there. Obvious holes such as one personality getting an injury which persists through the remainder of the film in a car wreck which wouldn't have happened if she were actually in the other unwrecked vehicle the whole time, plague this film.
This is laziest attempt of a director springing a twist on the audience that I have seen. Picture some dimwit introducing their self to you as Chuck, having a conversation with you, then proudly revealing their name is actually Aja and that they really outsmarted you. Yeah, quite a zinger there, Aja. When the rest of a movie becomes completely nonsensical due to the addition of a plot twist, perhaps that twist could have been left out.
The film looks as though they decided to appeal to the psychological thriller market ( a genre where this sort of stunt is painfully common) so they filmed an extra scene or two and tossed it in to let the audience sort it out. It's obvious the director didn't try.
In the end, nothing fits together. The events as depicted in the movie obviously didn't happen the way they were shown to the audience. What did happen? Did anything actually happen or was it a psychopath's delusion? What was the point? A film that leaves itself open to interpretation is not a bad thing in itself, but in this case the audience has nothing to interpret because they are given nothing to work with and there is little incentive to bother.