Too black, extremely violent

I will not say that the novel is bad. It is well built, and the psychology of the characters is interesting in the development of the enigma; especially the psychology of secondary characters, alas.
Indeed, thrillers are not my favorite reading. They are useful for me switch from one task to another. I enjoy generally a combined action with suspense and surprising twists, humor and a certain balance between "good and bad".
On these points I am disappointed. The author imagines crimes of rare cruelty and outcomes are unfair. One leaves annoyed.