If you grow up with the wrong values that give you a false view of the world, then that's what you will believe in. As much as she hated him sometimes, she had always felt sorry for him. He had simply been a victim of his upbringing. There wasn't always an excuse for the way he behaved today, but if he really had the Dark Mark, then it wasn't easy to get out of this circle, even if he might have doubts about his worldview.