Apt Pupil

What I thought would be a quick story, turned out to be a full grown complex novel with fully detailed characters.

In the beginning I felt torn between thinking: “Why is this boy harassing this old guy?” and “What's wrong with that old guy?”.

In the middle I could understand more of the relationship between those too, but I still don't get the changing of the characters. Not 100%.
Why did they go on and done what they've done? This plus some boring chapters and the fact that I thought this book would never end and dragged along endlessly, gives it 3/5 for me.