I'm not a lawyer, no.
The case suggested here is classic. Everyone should know how to react if they're caught in such a situation.
First, you have NO obligation to give any aid to your would-be attacker. In fact, doing so will hurt your case. How could you have been in mortal fear of this man one second and then felt such compasion for him the next that you administered first aid?
Second, you MAY leave the scene... if you don't feel that it's safe for you to stay there. This is not fleeing a crime scene, it is a continuation of your self-defense. Just proceed immediately and directly to the nearest place where a reasonable person would feel safe and summon authoriities from there.
Third, assert self-defense from the very beginning. When you make that 911 call, say, "A man attacked me, threatened my life, and I acted in self-defense by shooting/stabbing/kicking/whatever him.
Fourth, get your story straight. If you've got time to sit down and actually write it out, that's the best thing you can do.
Fifth, don't make detailed statements to authorities. If questioned, just assert that you are to upset, traumatized, etc., to make a statement right now. It's actually good to say things like, "I'm very upset. I'm not thinking clearly right now. I need to calm down." These sorts of statements establish a temporarily impaired mental state which your attorney can use very effectively.
Sixth, you will almost certainly be arrested. That's fine. Expect it. Don't panic. Arrest is not conviction. Remember, the police are just doing their duty. Be polite and cooperate physically.
Seventh, when someone says, "You have the right to remain silent," do so. When someone says, "You have a right to an attorney and to have an attorney present when questioned," assert that right.
Eighth, get a good attorney on your case immediately. I know it costs money, but what are twenty years of your life worth?
Ninth, resist all pressure to anwer questions or make a statement. A common tactic is, "I'm sure that this is just a simple misunderstanding. If you'll just answer a few simple questions for us, we can clear this all up easily and you'll be on your way." This is not a simple misunderstanding. A man has been stabbed. You are under arrest. It will not be cleared up with a few quick questions. Politely assert your right to an attorney.
Tenth, be very consistent in everything you say. Tell the same story every time. If you can't remember something, say so. It's ok to say, "It was a horrible experience, I was under huge stress, I don't remember what color shirt he was wearing. I was trying to save my life, not evaluate his fashion choices." If you're not sure about something, you can say that too, "I seem to remember that his shirt was blue, but I'm not sure about that, not sure at all."
Even if the poster in this thread is just making the story up, and we certainly have seen that happen here on bf.c before, it's still good for all of us to take this opportunity to review how to handle this sort of a situation if any of us does actually find ourselves in one in the future. Think of it as a fire drill.