You must be responsible for your own actions but you not responsible for the actions of others, unless you have a duty to be responsible, such as when you are the parent or the boss. While you must react to a situation and protect yourself or your family, you still have some responsibility for the damage you cause, so you must avoid doing unnecessary damage to an attacker.
Since most fights are an escalation from a minor disagreement, you should be able to recognize escalation and avoid it or diffuse it. Escalation is caused by both parties not backing down. When one party eases up, the escalation eases or stops. If you are escalating a fight, then you are provoking the other person into fighting and have no right to claim you were provoked. You can end most confrontations by safely walking away.
In a fight, the aggressor is not the one that throws the first punch, but the one who insists on fighting. If you ever get into a fight, it should be because you are forced into it. Take every opportunity you can to get out of it. Leave if you can, fight only if you must! If you do get into a fight, it is usually because you missed opportunities to get out of the situation or to assess the situation properly.