To be devil's advocate for your view: The Italian could have known whatever "secret" thing it was about Zidane that was sure to set him off since Zidane played in the Italian league for a few years. I bet he wanted to get a reaction out of him with whatever was said, but probably didn't think it would be a head-butt (from a butthead none the less).
yeah...
from continually looking at the picture posted above and the video on YouTube, I'm 100% certain he embellished it.
the only way that left leg kicks like that is IF he's stepping forward with the left at the moment of impact.
But he wasn't. He was in the process of putting the right foot down. All of his weight was on his left (back) foot at the time.
but yeah, it was a red card offense, no doubt. But it would've been without the stupid play-acting.