Post 19 was a very good response as to why Christians find this movie objectionable. The normal process in a fictional work is to use real settings of time and place and introduce fictional characters. When you introduce real characters you are at a different level. Post 19 used the holocaust as an example. How about a story about George Washington portraying him with inaccuracies? I think much of the stir about this work results from the author saying in some venues that it is factual, and then when pinned down in other venues saying that it is fictional.
Many many books are based on real events, and introduce fictional characters and twists on truth, or downright falsehoods. And if he believes some aspects of his book are true (as some believe the Illuminati and the Elders of Zion control the world), he'll say so.