A "dark comedy" that's just dark. Haven't laughed a single time and I love dark, morbid, cynical humour but this is taking it way too far. This kind of humour is a very fine line between good and terribly misplaced or simply bad. For this movie it is the latter.

The idea of showing a mentally ill person in a humourous way isn't that bad on its own, although difficult to not glamourize terrible acts or ridicule that person or the illness.
I am trying to be very general here as there are several kind of mental illnesses.
In this movie our main character has probably schizophrenia, I haven't exactly payed attention to the type of pills he's (supposed to be) taking or their label and don't think it was explicitly mentioned.
However, watching a woman getting killed by someone who is apparently mentally ill and persuaded by his cat and dog is nothing that I can laugh about and the movie tries hard to achieve that at times. Even with "accidental" killing.
I simply can't find anything funny about a person cracking the neck by "accident" and it is even worse when the maincharacter says afterwards something like "Oh, how could that happen", I've seen the German version, so I don't know the exact English words but you get the idea.
As a thriller this would work through and through, a simple plot escalating into much more, a "normal" guy becoming a serial killer who is extremely dangerous because of his mental illness and difficulty to perceive reality.

But the terrible attempts at the "comedy" aspect of this movie ruin the whole plot. Although I wouldn't really call anything plot here. There's no real end and the synopsis takes the first half away or less, the rest is dealing with the hallucinated world of the main character.
It is said that this is one of Reynold's best performances but I find it hard to support that claim considering the problematic direction of the movie.