Is a high level black belt MORALLY OBLIGATED to accept an attacker's surrender (aka tap out) in a real world street fight? Let me clarify with a senario. Imagine this condition: I attack you with a butcher knife & you being the high level black belt EASILY disarm me. I tap my leg three times and bow to you as a form of surrender. Do you respect that or no? Do you kill me anyway, hold me until the police arrive, accept the surrender and just walk away, etc.
A little clarification seems to be in order.
Morality has nothing to do with a street fight. Morality is the recognition of right and wrong and their application in every day life. A street fight has already started with the rejection of right (using the common societal definition, I don't want to get into a philosophy discussion) by at least one person, so why would you expect the outcome to follow ring style or other idealistic system rules?
A second point: If you are the black belt in this scenario, and you follow through on the disarm and kill them scenario, then at least in the United States, you can (and probably will) be charged with murder. As soon as you are at a point where you can safely disengage and don't, then you become the aggressor in the scenario and can be charged accordingly.
So to answer your base question, there is no obligation on either party in a street fight. This isn't feudal Japan or anything close to it, there are no liege lords, etc. It is a street fight, plain and simple. Do what you have to in order to get out of it with the least injury, and be prepared to legally defend your actions if need be. That's about it.