What is the story of Marduk trying to teach its people?

The Story of Marduk does not have any particualr moral teaching at its core. Remember that a myth does not have to have a lesson, and can be just for entertainment. In this particular instance, the story of Marduk is one of many stories about the eventual destruction of evil by good. This is one of the most common story motifs of all time. Everything from The Bible to Star Wars.