What is the theme in The Dark Tower: The Gunslinger by Stephen King?

Revenge is a central theme in the book. Initially it seems that revenge is the motivating force behind the gunslinger's twelve-year pursuit of the man in black. Early in the story, the reader is not given much information why the gunslinger has this need for revenge. Only the gunslinger's strong feelings of retaliation, anger and unwavering pursuit are described. As the book continues, connections are made between the past and the present which more clearly explain the relationship that the gunslinger has with the man in black and why he feels the need for vengeance.