Its interesting that you use Robin Hood as he really is great example of audiences wanting to see things a certain way. One of the classic things about the Robin Hood stories is the wonderful King Richard who is away on the Crusades, but if you look at history Richard was a truly horrible King. He grew up in France, only coming to England (which he didn't like) when he became King. He took all the money from the country leaving it bankrupt, using it to fund his crusade. Then when he ran out of money he came back and massacred the Jew of York for their gold to fund another crusade. Yet in the Robin Hood stories he is the brave and wonderful "Richard the Lionheart".
So "realistic" Robin Hood story would probably have King Richard as the villain, not Prince John, but that will never happen as that is not what audiences expect from the story. People are used to the legend, and to steal a line "When legend becomes fact, print the legend".
Now this is a little bit harsh on Richard.
He did grow up in France, and he did speak mostly French, but it is important to remember that Normandy was part of the kingdom back then - and Richard's family was the one descended from William the Conqueror, duke of Normandy. Most of the nobility did not speak English either. Henry II, widely considered one of England's finest kings, also spent most of his time on the continent and ruled a French court.
And although his crusades and subsequent captivity emptied the coffers of England, it is also true that he was a marvelous military leader in the Cruzades, accomplishing much more than any of his European counterparts. He is described as being extremely brave and charismatic, and he did manage to strike a peace deal with Saladin in the end. Was he cruel from time to time? Yes, but so was Saladin, and we don't look down on him because of it. They were products of their times.
John is considered the villain not because of being inherently evil, but because he did attempt to usurp Richard's crown before Richard's death. Treachery could be forgotten - but it is not like he fared much better once he got the crown either, with the Barons revolting against him and forcing him to sign the Magna Carta. John is the antagonist to Robin Hood because to the noble class, he was antagonizing. Richard was a non-entity, being away all the time, and propping him up makes John look even worse in comparison. It's simply the way things worked out.
Historical debate aside, I agree with your sentiment: people become attached to an idea, even if that idea is not originally true. In the context of BvS, Superman might have been concocted by Jewish creators, and might just jump tall buildings and throw off a sarcastic prank every once in a while, but that's not how he achieved longevity. The version of the character that has endured is the god-like boyscout that everyone remembers. The same goes for the original Batman killing. It may have been a trait in the beginning, but nobody cares, because that's not how the character is remembered.
Delivering the unexpected to audiences can be received positively, of course - but it can also crash and burn just as easily, as we see by this movie's deplorable legs in the box office.
