This whole attempt on Trump's life reveals so much about his personality.
Instantly, he recognized that this was a photo opportunity, and a chance to look strong. He is always TV-ready (that's why he has the orange makeup, he looks healthy on certain kinds of TV cameras), and always mindful of the cameras the the PR opportunities afforded by visual media.
And he knew he needed to say SOMETHING when he was fist-pumping in defiance. What did his instincts tell him he should yell? "Fight"