Most misunderstood film I've ever seen on IMDB *spoilers*
The monster isn't literal. The film is about addiction and how it can destroy people and the people around them. So many reviews here talk about how the flashbacks are boring and break up the action. You're watching a drama as much as a horror film. The reason you can't engage with the film properly is because you don't seem to understand what you're looking at.
There are actually people here asking where the monster came from. How about the film "Carrie"? How come she has all those mind-powers and no one else does, either in the film OR in real life? That doesn't make any literal sense! What a dumb movie!
If you don't understand this film and others like it then coming on sites like this and complaining about how bad they are really isn't doing yourself any favours.
Please go back to your Hostel movies and stay there. Or you could look up the word "subtext" and spend a few hours trying to puzzle out the meaning. Good luck with that.
For the record the two lead performances and direction are top notch, as is the cinematography, and the practical effects are impressive considering the budget.
It's an intelligent, well-made film, and I'm glad I watched it.
____________________________________
"...needless to say, I had the last laugh."