Understanding book 9 of The Republic

Hi...I'm trying to understand a concept my teacher said would be on our next quiz which is on The Republic, by Plato, Book 9.  The question I'm trying to make certain I get is:  why is the unjust man more miserable than a just man?  This seems easy but when I read this book 9 Plato is talking about a terrified slave, and that the tyrant (miserable man) is unable to really judge true pleasure and so he can't know pure pleasure...well, I don't know if I'm really digging deeply enough...probably not, could anyone explain this concept to me?  Somehow I never trust my own judgment re this class!