What is the theme in Water Margin by Shi Nai'an?

The fall out from a corrupt government is one of the major issues dealt with in this novel. The reason that the bandits have formed their coalition in Liangshan Marsh is because they have been wrongly accused of crimes, or have been forced to commit crimes in an effort to protect themselves or others. For instance, Song Jiang kills Poxi because she is trying to keep a letter from Chao Gai that could endanger both Song and Chao.