Books in which the characters express negative emotions--or even commit crimes–can console those who have experienced similar emotions or have committed–or merely considered committing–a crime.
That is the beauty of literature, the escapism and exploration. You can experience crime with out committing it, just like riding a rollercoaster feels like you are experience a near death fall. It also shows us that we aren't alone in our feelings. When we read that some else is having the same experiences and emotions we don't feel so isolated.