It's worth noting that the book Joker grows out of his obedience to the empire and gets to know and relate to the people he has come to Vietnam to kill. The movie doesn't explore the second book of the duology (despite borrowing some imagery from it), where the consequences of serving the empire come back to bite Joker in the ass and, ostensibly, make him a better person.

All in all, I like the books a whole lot better.