It can get pretty complicated, but generally, if he was determined to have been the aggressor (by taking the sign in the first place), then it would be very difficult for the security guard to show that he was justified.
Something like this would have to be sorted out in court. But, given the relatively minor nature of the altercation all around, I doubt either side would actually press charges. So it’s all likely hypothetical.
Thank you for your insight.
I’m not so sure that the protestor won’t press charges, considering the political nature of the event.
But, we shall see.