I think Zidane knows very well that it's an international game, that it's his last game, what he did was wrong etc... He's been playing so long. He cannot possibly NOT know all this.
I'm not condoning his actions, but I think that irregardless of whether he's a player on the pitch, captain of the team, talisman of the French team and hence expected to be disciplined and whatever, he's still human. If Materazzi was really being an ass and really ragging on him in a really bad way, I can completely understand why Zidane reacted the way he did. Few can actually walk away from insults hailed on them by opponents without snapping, especially when they are already under the heavy scrutiny of others while bearing a lot of pressure on their shoulders to perform up to and beyond people's expectations.
But really. This is all conjecture at this point in time. Nobody really knows for sure as to what it was that Materazzi said that caused Zidane to explode and the both of them are keeping quiet about it so... *shrug* I'm just sad that because of it he missed out on the rest of the game that's all.
That said, I did find it highly amusing that Zizou chose to use his head and not his fists. XDDD