What hasn't been talked about here, and it needs to be, is the idea of "escalating violence." Although they can be this way sometimes, violent encounters generally don't start with a guy trying shot you in the head, or cut your throat from behind; unless their intention is to assasinate you. They usually escalate from an initial incident, which will require an appropriate response. Too much or too little of a response will escalate the violent encounter, thus being poor self defense. An appropriate response may prevent the encounter from escalating.
Example:
#1 Violent encounter: Green Bay packers fan yells accross the parking lot at you, "Lions Suck!" You are wearing a lions shirt.
Appropriate response: Ignore him. He isn't in your way, and he is too far to be able to tell if he is joking or not, or what his deal really is. Just keep walking to your car.
#2 Escalation: He now is approaching you rapidly and yells, "Hey Mother F-er, I said the Lions Suck! You got a F**in' problem with that!"
Appropriate Response: Assume that you are not close enough to your car yet to simply hop in and drive. There are no people to intervien. Your not by your vehicle yet. It is clear that he is looking for some kind of reaction from you, and that he will not stop until he gets one. So you feign your 'submission to his superiority' by saying something like, "Hey man. I don't have a problem with you, and I want any trouble." You have turned to face his direction a bit, while still walking a bit slower to your vehicle. Assuming your without a firearm, you have pulled your tactical folding knife and are concealing it in your palm.
#3 Escalation: He responds with "F-You...I think you do have a F-ing problem!" And you realize that he has 2 friends with him, following behind. They are coming towards you even more rapidly now, and are in fact almost on top of you.
Appropriate Response: You run, keeping an eye on them in your perephials. The car is closer then any other people or lights would be, so you run towards your car. You have also unfolded your tactical folder. You are still concealing it, but your weapon is ready if need be.
#4 Escalation: You hoped that they wouldn't chase after you, but they have, and are gaining. You get to your car, but they are far too close for you to get inside without getting ambushed.
Appropriate Response: As soon as you are by your driver side, you quickly turn and yell, "GET AWAY FROM ME!! I am asking you not TO COME ANY CLOSER!!"
#5 Escalation: Your new friends stopped running when you did. They say, "Oh, your going to get the beating of your life now!" And they start moving towards you, into your space.
Appropriate Response: You now brandish the knife, and cut the nearest thing, the hand of one of your attackers. You say with force, "I'm telling you, you guys do not want to do this!" You keep an eye out to see if any of them look as if they are going to pull a weapon of any kind, so you can cut them fatally.
#6 Escalation: The attacker screams, "You f**ing cut me! I'm going to kill you!" and he really makes a break for you, as does his comrades.
Appropriate response: You know start cutting more vitally and rapidly, maintaining distance to prevent a grappling circumstance. You pop one of them in the eye, and he goes down screaming. The "spokesperson" of the group, after being cut a few times decides it isn't worth it and runs; his friends follow.
End of situation: You go to get in your car to go. A couple comes over to see if you are alright, and they say that they called the cops on their cell phone. They saw you running from them, and you made it evident by your actions that you were the defender and not the aggressor. So you decide to wait for the police, and the couple decides to wait. You would normally leave the scene for safety, but a squad car can be there in less then 5 minutes to take the report. You now have witnesses that you were defending yourself, so if the attackers decide to say that they were attacked by a crazy person with a knife, you have proof otherwise. Your @$$ is covered legally, and you are alive and unharmed.
Moral...what am I trying to point out here?
Yes it is true, there are no rules on the street. In this circumstance, "you" did not fight fairly. Yet, "you" maintained acceptable force throughout the situation. You did not turn into the aggressor. It just so happends that the hypothetical situation continued to escalate. Most situations will not go that far, as long as your prudent in your response. You don't want to fight with a "moral code", but you want to be prudent, and recognize how force escalates.
Would it have been prudent for me to start running towards him like a madman with my folder drawn after Action #1? Or how about #2 or #3? No! The attacker may not have taken the circumstance past action #1 or #2, but by responding aggressively rather then assertively I become the person that escalates the situation. I may hurt someone unessicarily, endanger myself more-so, or even worse, I may come accross as the aggressor and a witness may say that I attacked them with a knife! An inapproriate response = poor self defense.
Do you see the point I am making? Do not fight fairly. But, recognize how violence escalates, and what a prudent and appropriate response would be to prevent this from occuring!
If I'm in a bar, and an intoxicating person attacks me, I usually don't strugle! :rofl: 
