In the world of the armed citizen, the best rule of thumb is ADEE - avoid, de-escalate, disengage, escape, evade. Your situational awareness and ability to spot trouble in advance and avoid it - a major combat skill - is still critical.
You ADEE as much as possible.
But when someone brings it to you in spite of your best efforts and you can't avoid it, then it's on. You don't get to START it, but you'd better be fully prepared to FINISH it -- up to the point your attacker disengages, surrenders or is rendered incapable of further attack.
Legitimate self defense ends when your assailant's capacity or will to attack ends...
Take a look at
http://www.amazon.com/Meditations-Vi.../dp/1594391181 and see if it helps. Don't worry too much about the 'martial arts' in the title, it's the real world violence stuff that counts. Or see his blog at
http://chirontraining.blogspot.com/ . Stuff like this:
Any sport is a contest designed to test like against like and to measure the things we value as a species. So sport measures strength and speed and endurance (just like any animals trying to win a herd of females) and certain elements of thought. It does not measure and specifically forbids finding out who can go murderous quickest, who has least social conditioning, who can cheat first and hardest... All things incredibly valuable in every other type of physical conflict.