This canon features an abusive relationship. That's not a problem, in and of itself: I think everyone agrees that featuring bad things in fiction is not necessarily wrong. The problem is that Character A, the abuser, is the primary antagonist, while Character B, the victim, is a significant supporting character. But B does not get an arc about breaking free from abuse, joining forces with the good guys, and taking down A. Instead, B chooses to sacrifice himself to save A. As a result, A completely rethinks his worldview and switches sides in the end, allowing the heroes victory and ending a corrupt system. B's character ultimately serves A's character. In other words, this work features an abusive relationship in which the victim dies to spur positive development on the part of the abuser. Most people would agree that this is not okay, and it sends a negative message that reinforces the essence of the kyriarchy: the feelings and character of the oppressor are more valuable than the feelings and character of the oppressed.
But the source material is really good, with ultimately positive themes about the role of family in life, the necessity of growing up, the part of free will in maturity, and the importance of choosing humanity over the system and people over ideals. A and B's relationship serves its plot, character, and themes perfectly. A is motivated in the particular case of this relationship not by machismo, insecurity, or displaced anger, but by a calculated strategy that's half combat tactic and half deathwish. The relationship is acknowledged within the canon as a flawed and tragic one in which A ultimately owes a great debt to B. A in fact states, as he's dying at the end, that he had to make the redemptive choice that led to his death or he wouldn't be able to face B (as well as another dead character who also chose to save his life, much earlier in the series, but who had a reciprocal if imperfect friendship rather than an abusive almost-romance with A). Does that make it okay?
Some people might say yes, but it's not that simple. While a reading of the text in line with the creators' intentions would have B choosing to stay with A and sacrifice himself for him of his own free will rather than because of the psychological manipulation A uses, it's possible to interpret the text differently and believe that B was brainwashed the entire time. In which case, it's definitely not okay. You could say that that interpretation flies in the face of all the text's themes and the creators' intentions, and that's true, but does that make it wrong? And even a best-case, canon-compliant reading of the text brings us back to the fact that the victim's character serves the abuser's character arc rather than the other way around, reinforcing the values of the kyriarchy.
So am I sinning¹ by enjoying the relationship and its source material? Did the original creator sin by conceiving of it? Should she have reworked the text to find a different way to convey her message, even though that would have resulted in a work less appealing to me personally? Did the people who worked on the various adaptations² sin by including this relationship? Should they have reworked the text to remove it?
When you say works of fiction should be socially responsible, where do you draw the line, and how do you judge the people on the wrong side of it? What about works that are intended to have a positive message, but can be interpreted negatively? What if the work is well-constructed so that all of its aspects support the more positive themes, but some people still take away a negative message and are upset and offended? You can't just tell them they're "wrong," even if their interpretation isn't supported by the text, because that would be denying their right to be offended and privileging the author's interpretation over the audience's.
So what do you do? Do you stop enjoying a work because it's problematic or could be interpreted that way? Do you refuse to engage with fandom for it? Do you refuse to produce fanworks for it? Is it enough to acknowledge the problematic material, or should you take steps to condemn it? If the latter, how far should those steps go, and how much of your personal enjoyment should you sacrifice to take them?
¹ I say "sinning" because I don't want to say "am I a bad person for enjoying it." That implies that doing one wrong thing makes someone a bad person, thus setting up a false dichotomy and inviting reassurance rather than honest answers.
² I am working my way through the original text, but I remain most familiar with (and, so far, loyal to) a much more recent adaptation.