Justice demands the opposition of injustice. The harming of an inoffensive person would seem to be an injustice of the grossest kind, whether that person is yourself or someone else. If you allow an assault to proceed unopposed you become an accomplice, and share the guilt. We are all connected. It is bounded by that form of ethic known as appropriate force.
Appropriate force is the gauge by which a response is measured.
The parameters are the violence and perceived intent of the attack. An attack that is clearly meant to intimidate or annoy obviously does not call for the same level of response as a murderous assault. Appropriate force is that which will effectively repel the attack without radically escalating the level of violence involved. Not enough response will worsen the situation, too much will place the attacked person in the position of being an attacker. Against nuisances, intimidators and most other attackers, they are decidedly inappropriate.