He already did!
Gibson already did state, loud and clear, that the Holocost happened. And when he "dared" to bring up the fact that other people - people who weren't Jewish - were slaughtered during WWII, a prominent Jewish official (I forget his name) attacked Gibson. Mel cannot win. Not when people who are obsessed with "political correctness" are involved. Mel made it clear in his latest "Reader's Digest" interview that the film is about love, and the greatest sacrifice a person can make because of their love for others.
And for your information, everyone was involved in Christ's death, including the Romans.