My actual rating would be 3.5 because this is not actually a bad thriller. In fact, the idea and the story are quite good. It's just that the the writing didn't quite work for me, although I can't quite put my finger on why that was the case. It was too easy for me to put the book down without any urgency to get back to it, which means it didn't really work for me as a thriller.
Having said that, this was a first book by this author, and it does show a lot of promise. If he publishes a second thriller, I will definitely give him another go.