Ah, I see. I understand now. You have a problem with using context for comprehension across the board. Because the way you have characterized my argument? Completely out of context.
I did not say the line is okay because it's how BvS portrayed Superman. I said the line means something different than what you are saying it means based on the context of the rest of the film. "No one stays good in this world." The rest of the film contextualizes the line as one that means that no one stays "all good" forever. And, it's true, as Bruce's final monologue explains: "We fight. We kill. We betray one another. But we can rebuild. We can do better. We will. We have to." BvS, as a film, is making the case that "good" is not a status quo that, once lost, cannot be recovered. Redemption is possible. What falls isn't fallen, as Bruce wrongly believed at the start of the film. Superman's "No one stays good in this world" is an acknowledgment that he may have to do something imperfect -- something that challenges his goodness. But he is not saying that no one can ever be good or find goodness again.
You cannot judge the meaning of the line out of the entire context of the film. It was not said in isolation. It was obviously said to contribute to an overall theme or thesis. And the film's overall theme or thesis is that what falls is not fallen. One may not "stay good," but one can always find one's way back to the light. Good is a conversation -- an ebb and flow -- and not a promise one can keep.
I do not have a problem with such a different interpretation of the character because Superman, in that moment, is no different than every other interpretation of the character. Every version of Superman does not believe that everyone can be good all the time. Every version of Superman does not believe that those that fall are fallen. In the recent Action Comics #1000 issue, Geoff Johns revisits the first issue of the comic in which Superman saved Lois from Butch, a criminal and a sexual harrasser. Superman reaches out to him because he believes that even though Butch has not stayed good, he can be good again. And, by the end of the comic, Butch seems like he's heading down a different path. 
"No one stays good in the world" is an important and vital lesson for Superman and anyone to understand. Because no one does stay good. We may be born with a tabula rasa or an uncorrupted soul, but we will make mistakes. We will sin, fall, and do bad things. But those things do not have to set our fate. Not staying good does not mean staying bad. Which is to say that the point you were making was dead wrong because your point was that the line, and how it characterizes Superman, is apocryphal: it isn't something any Superman should think or say. It isn't something Superman should think or say if you only heard him say it out of context. When you look at the bigger picture, it puts his words into a broader context that changes the meaning you are ascribing to it.
I also want to point out that often when Superman killing Zod in Superman II is brought up, people say it's fine or works better because of the context of the whole movie's tone and reception. Likewise, Hippolyta is just as if not worse in her attitude toward Diana's heroism than the Kents, and Diana says incredibly pessimistic things about humanity, even essentially disowns it at one point, but there is no such uproar. In short, I think you're mistaken and unfair in your analytical approach. Characterization is made up of more than one line of dialogue, and in this case the line isn't even one that reflects poorly on Superman.