Everyone remembers the big to-do on the Oprah show where she laid in to James Frey on the facts of his autobiography, A Million Little Pieces. Her argument with Frey was that he had cheated his public, that he had lied to them about his own life. Where they thought they were getting truth and reality, they got fiction instead. She felt personally betrayed because she had included his book on her book club list.
I don't deny that Oprah did the right thing. In fact, I think he got exactly what he deserved. The line between fact and invention has been blurred terribly, so that fiction writers like Dan Brown can mess with history and get away with it because he calls it fiction. And yet, what he does in The Da Vinci Code is exactly what Frey did in his memoir. Moreover, what Frey did was lie about himself, while what Brown does is lie about us. Our past is what we are; our past is what makes us up. Playing with the past is like Russian Roulette--sooner or later, the chamber is going to have a bullet in it.
This does not mean that I am against historical fiction. I love it, and plan to write some myself, but I think that your fiction must be woven into the facts, not slice through them so you can invent your own history. For example, in Angels and Demons, Brown says that Galileo was the founder of the Illuminati, which is outrageous, since the Illuminati were an Enlightenment invention, out of Bavaria, not Rome. The name speaks for itself--Illuminati--Enlightened Ones.
Now poor Galileo has been mythified one more time. If anyone want to know about Galileo Galilei, there are tons of readable histories about his life, and not of them mention the Illuminati. In fact, the reason that Galileo was under house arrest at all was because he refused to go to Germany or England and join the Protestants. He was a Catholic to his bones.
It is up to us as writers, especially those of us who write about the past, to do so with respect, to get the essential facts correct, and not impose our own egos on our ancestors.