This argument is invalid for two significant reasons:
1) Gibson went to great pains to claim he was making the most accurate depiction of Christ's suffering ever created. Some viewers are even claiming it's like a documentary, a judgment which reveals either a lack of sophistication (fine) and at worst a form of willful ignorance (not). I've seen a good 15, 20 minutes of the flick, and it didn't appear remotely like a documentary to me.
2) Filling in gaps with fiction is ostensibly fine, but it's also fine to argue there's an agenda when an identifiable pattern appears in those fictionalized additions: Why is Pilate presented so sympathetically when he clearly historically was actually a brute? Why add scenes of Jesus being physically brutalized by the Jews when the Bible mentions nothing of that? Why does Gibson choose to have the High Priest mock Christ, telling him to come down off the cross if he's the Messiah when that person isn't specifically identified in the Bible?
Step outside of the film and look at how Gibson has responded to questions about his beliefs on the holocaust. When asked about his father's beliefs, he responded that ""My dad taught me my faith, and I believe what he taught me. The man never lied to me in his life." What the hell does that mean? Especially considering his father's an avowed holocaust denier. And why won't Gibson come out clearly on the Holocaust instead of beating around the bush about it? Peggy Noonan asked him about the Holocaust and here's his mealy-mouthed response:
"War is horrible. The Second World War killed tens of millions of people. Some of them were Jews in concentration camps. Many people lost their lives. In the Ukraine, several million starved to death between 1932 and 1933. During the last century 20 million people died in the Soviet Union.”If you've ever heard arguments from holocaust deniers, their MO is to diminish the scope of the Holocaust by saying fewer Jews dies than reported, and then they invariably conjure up Stalin and the Soviet Union next - sort of a switcheroo to change the subject. Gibson's evasive answer is chilling in its similarity to the deniers. (Note that although he mentions millions dying in WWII, he doesn’t mention millions of Jews specifically, just “Some of them were Jews in concentration camps.”) I’d like to think he’s not a Holocaust denier, but I really believe the jury’s still out on him. Someone really needs to question him better. Specifically.
Point is: You can’t consider the fictional additions Gibson has made to his film without considering the attitudes and beliefs Gibson brought to the table.