This is a reasonably good read, and but personal preferences weighted more on my assessment than the "technical" quality of it. 
 Why I liked it... 
 - I kept on reading, even though I wanted to throw it out; 
 - It's a story about the worst in people and provides quite a philosophical debate on human nature; 
 - It's well written: the plot is well tied and the writing styte is instigating. 
 Why I didn't like... 
 - I don't like mysteries where the mystery is gone half-way through; 
 - Both Amy and Nick were detestable in character and actions, hard to cheer for or even sympathise with; 
 - I never enjoy stories that make me hate the characters on principle, which this one did.