Someone is trying to kill you or seriously injure you. Even if the intent is not to kill you, the person assaulting you is trying to deliver as much force as possible to your body. He is not feeding you a rule-based fighting technique or combination that is designed to create an opening so that he can score a point or knock you out. He is trying to disable you, he is beating you down, his intent may be to kill you. Unlike sparring he is not holding back in order to stay within the rules. The attack is not fair, he tries to get the advantage over you through the use of shear violence until he decides it is over. There is no referee to stop the attack. It is over when he decides it is over. When the threat gets you into a headlock his intent is not to get you to tap out, his intent is to choke the life out of you. No amount of tapping out will make him stop. He my get you in a hold to throw you over a balcony or to slam your head into a wall, not to get you to submit so that he can score a point. You need to have the mentality to fight back in exactly the same way. Taking the honorable route and only using “fair” strikes will encourage the attacker. Being afraid that you will hurt the attacker if you do what you have to do will not work favorably for you.
You do not have the luxury to study your opponent before the time like in sports fighting. You have no idea what he is capable of doing. You do not know whether he has a weapon or whether he is willing to use it. There is no protective gear. It is just you and him and there can only be one survivor, not one “winner” but one “survivor”. Sports based fighting are dominance contests. Assaults are survival contests. In self-defense your goal is to stay alive and escape. In sports fighting your goal is to win the match.