Self defence is just that, and you shouldnt worry about the outcome if you believe you are in trouble. Just use your head and dont go overboard once the incident has been stopped.
In New Zealand "assault" is also the intention to injure, so by saying "Im going to knock you out" you are entitled to defend yourself. You dont have to get pushed/hit before you defend yourself (or another). What gets people into trouble is that once the situation has been stopped, the defender is still adrenalised and continues to pummel the asshole into a hospital bed. You have to use the minimum force required to stop the incident, and that is dependent upon factors such as size, aggression, multiple opponents, skill of opponent etc, you cant go Jet Li on a nerd, but on a biker gang you can go mental. You also cannot pursue them, if they run off, move on, report it, but dont go hunting.
Fuck the judges who know nothing about MA. If the MMA guy that was mentioned started it, far enough, but if he defended himself, well good. one less road rager wank to pick on others. I guess he went beyond the force needed to stop the aggressor.
I have info on NZ law regarding assault, but you others will (and should) investigate these important laws in your area. There is alot of info on it, but narrow it down to Summary offences & breach of the peace (citizens arrest) areas. My associates and I havent been convicted at all, since we taught ourselves the law, we know when to hold off, and when to unleash, and in this simple know how: knowledge is power.
It should be a part of your training. know thy enemy & know thy laws. Learning them takes the guesswork out of it. makes you more confident and competent.
If you want a view of NZ's laws, just in the small areas concerning us, drop me a line and i will email you some of our info. It will be different in each country but im sure the idea will be similar.