He had to reasonably believe that he was in imminent threat of death or great bodily harm. The test is, would a reasonable person feel that way under the circumstances? It is difficult to determine reasonableness one way or the other without more facts, witnesses, or a video of the crowd in front of him at the time. This is not like a Trayvon or Michael Brown situation. But all self defense situations arent like that either.
If he had watched other Trump rallies when protesters attracted Trump supporters, then yes, he was justified.
One of the “protesters” was wearing a mask. How was he to know they weren’t going to lynch him? I am not necessarily defending his action of brandishing a weapon.
If you noticed the video specifically did not show the crowd. I would like to see the video that he shot. This will no doubt be appealed.