As an citizen, we probably won't get to decide the circumstances of the fight if it ever comes to us. Numbers, location, lighting, motivation, and level of force required will almost certainly be out of our hands when the fight starts.

Recognize the threat as soon as possible and react with the minimum level of force necessary to end the confrontation. That might be a smile and an apology for something that wasn't your fault. That might be lethal force.

If you decide that violence is the only viable option, don't ease into it. Go ugly early.