So, you're watching two guys argue. You are the only person around. One pulls out a knife and stabs the other, before driving off. What you are telling me is that you don't think you have a social obligation to call an ambulance, is that right?
Yes, that is basically right. Although, my personal opinion on it is irrelevant.
Well, I guess I have to attack this from a different angle then.
I don't think the right response is to make a law.
Telling you you are signed into a social contract means you cannot get out of it except by leaving society, and furthermore, you never formally agreed to it. So that angle is a no-go.
I also don't believe you should call because you would want others to do so if you were in that situation, because all you would have to say in response is "I am selfish". Its also an emotional appeal anyways.
You're probably not religious, so that angle doesn't work.
Even so, leaving a person to die when you can help is surely a moral evil.
That said there is something I can impose on you without your consent, and I have every right to do so, even if you don't like it. If I knew you personally and you shared with me this opinion, I have every right to discontinue association with you and there is nothing you could do about it. I imagine many others would feel the same way and this would lead to you either deciding to change, or become lonely.