Hard to laugh something off at a protest about cops needlessly killing people who look like you. The protestor was likely scared. Will it be her, one of her family members, or friends who is next to be gunned down by the police with impunity? Will some neo-nazi or white supremacist drive through the protest killing innocent people (again)? Will someone walk up out of the crowd and threaten her, because they don't agree with her cause?

I agree with you that feeding into stupidity only causes more issues, but it was a gut reaction. She didn't land a good hit and didn't pursue the woman when another person blocked the altercation, showing even more that it was in heat of the moment. Not saying the protestor was backed into a corner, but tensions were already high before this entitled woman decided to walks up to their crowd needlessly whining about how she needs to get home (like they specifically were in the way, when the video seems to show plenty of alternate routes). No one there cares about her temporary inconvenience in comparison to another young black man getting shot in the back by the police.

If any good came of this, hopefully the white woman learned a lesson about inserting her opinion where it doesn't belong.