Sorry, but I believe you are entirely wrong. Even the police are investigating it as aggravated assault by the right wing idiots. If the professor felt threatened (especially due to being harassed), he was within his rights to defend himself, even if that meant he made the first move.

He was outnumbered two to one, and they were stalking and verbally harassing him with the intent to incite a reaction. Additionally, that kind of speech is not protected. If anyone should get charges, it’s the two abusers.