That would be a combination of the norepinephrine and testosterone. Mostly norepinephrine. The flight or fight response is caused by norepinephrine and epinephrine. Whichever you have more of determines which you do. When you have more epinephrine (fear) your brain is releasing that because it has determined that the person or thing that is threatening you is something you can't win against and could endanger your survival. This is the flight response and you leave the situation. When you have more norepinephrine (anger), your brain is releasing that because it has determined that what is threatening you is something you can handle and beat. This is the fight response, and it causes aggression which makes you want to hit something. What you actually want to hit is the thing threatening you. However, when this is just a situation that's making you angry, it's impossible to physically hit a situation so you hit something else. When it is a person that's causing your anger, you can hit them, but most people consciously know it would be illegal and socially unacceptable to do so, so you hit something else. Sometimes, people are too angry to consciously care, and they do hit the person making them angry.