My biggest problem is that the movie doesn't seem to work one way or another. Although I don't know the book, I can imagine roughly two ways to approach a story like this: a thriller like Loft, or a black comedy like Quiz. The first one would have worked best in my opinion, given the grave subject matter of teenage homicide, but director/screenwriter Menno Meyes seems to have been aiming mostly for the latter. That doesn't have to be an obstacle for a good movie, but he doesn't have a firm handle on the material to make it work like a black comedy with some thriller elements. Maybe also because he only allows himself a meager 88 minutes to tell this story, which leaves little time for background to the characters, or a decent ending.
The story is partially about the interaction between 4 characters, and partially about the interior monologues of the main character Paul. It is therefore not surprising that the book was adapted into a successful stage play first. You can almost picture how the parts where Paul breaks the fourth wall were done in a theater, with a spotlight turned on him alone, and the other cast members kept in darkness. However, what works in theater doesn't necessarily work as well in cinema, also because it is only done a few times and not consistently. The rest of the times, Paul (Jacob Derwig) just provides voice-overs, which started to annoy me after a while. This is an obvious literary technique that can work in books and on stage, and sometimes in film. However, when constantly abused in a movie, it feels like a lack of creativity in using the visual medium; after all, one of the golden rules for film is "show, don't tell". Other advantages that movies have, such as building tension through photography and editing, are sadly underused as well. Director Meyes' choice not to rely too much on cinematic techniques can be praised, but he puts too much faith in the monologues, dialogues and the actors.
Which brings me to the final point of criticism: the characterization of the four protagonists falls flat on its face. Character interactions, especially in a black comedy, work best if you have actors who know what makes their characters 'tick'; just look at Roman Polanski's 'Carnage' for a good example. But Jacob Derwig is the only one who gets a bit of backstory, and even so, his character is merely annoying and frustrated instead of interestingly layered. The other three characters range from sad to reprehensible, which could be interesting if they weren't so one-dimensional due to lack of exposure (apart from what Paul tells about them in his endless voice-overs). They often act and respond completely unpredictable, not as a twist, but simply because of poorly written roles that undoubtedly left out too much from the book. The talent of these fine actors feels quite wasted here, because there should have been more time allotted for exposition.
Every adaptation needs to omit certain parts of the narrative, but here it feels like most of the heart of the story was cut out. The basic premise is still there, but the director did not have the experience to make it visually interesting. After an Italian remake, there has been talk about an American one, but it seems to be stuck in development for some time. Let's hope they use that time to get the script right this time, because with the right basis, this premise could really make for a good movie.