In a fight, violence of action typically wins the day. You need to become violent enough fast enough to end any fight as quickly as you can. The longer a fight goes on, the greater the chance that you will get hurt. So, if you end up in a fight, it is your goal to end it quickly. Put your opponent out of the fight using only the amount of force that is reasonable for the situation.
In this video, the man leaving the elevator could have just walked away, but he turned and attacked the other three men. In this case, the man is committing an assault. He is the bad guy. But bad guys know how to fight, and they will use violence of action to overwhelm your defenses. Be prepared for that, and be ready to counterattack to the best of your ability with greater violence.
Nothing here really rises to a situation that requires the use of deadly force. It is a fistfight (though fistfights can also be deadly). Still, this guy, while a much better fighter than the other three guys combined, is not acting as though he is trying to kill anyone. He is just administering a beatdown. He wins the fight largely because of the violence of action he uses to keep his opponents on the defensive.
Get Out of the Way
Finally, do not be that guy in the background. If you walk into a fight between two other people, get out of the way. Because you do not know what is happening, you should not pick a side, but you should certainly get out of the way.