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"
Stripped away from all other thoughts as a bullet struck his ear, adrenaline pumping, logic and reason gone, fight or flight mode, all his brain could think of in that moment was "fight"
Fight what? The shooter? He's dead? The democrats? Guns? It was meaningless, just a thing to say, and his basest instincts told him "fight" was the thing to reach for.
In the immediate aftermath of violence, all he could think of was more violence.