I understand what you mean, but I don't believe that's really an issue given the context of what is happening in the story. I think it's a case-by-case thing.
I have no problem with emotional manipulation if it's executed very well. And Oda almost always executes it well.
There have been plenty of times throughout the story where Oda has written a scene that makes the reader feel a certain way towards a character without it coming off as forced or manipulative. Those scenes can tend to be important moments in an arc that sway the events leading up to the climax.
I acknowledge that what I'm describing is pretty subjective. I don't agree that it's well done, but I think it's totally fair for you to feel otherwise.
Like I said before, storytellers aren't neutral. They arrange scenes and panels and music and composition to make people feel a certain way in certain scenes. I guess for me, one important trick is to make sure that the audience doesn't NOTICE that their being manipulated, and....well, with OP, I frequently do.
The scene wasn't really about making us hate Niji as much as it was showing us how different Sanji's worldview is from his brothers, making it all the more satisfying when he eventually beats them down or if there are character changes down the road. Keep in mind this is also a chapter that told us about Zeff being captured. It was about setting stakes: This isn't just a situation of life or death for Sanji, it's a situation of one ideal clashing with another.
I definitely think the larger point is that we're supposed to hate Niji, you're just phrasing it in more sophisticated terms. Which is legit, but you're basically saying that we have two antithetical worldviews, and when Sanji's beats Niji's, we're supposed to feel good about it. Why would we do that? Because Niji's worldviews are all but explicitly stated to be wrongful. So, basically, since Niji is wrongful and Sanji righteous, it's implicit we're suppose to hate Niji, right?
That said, it doesn't help that I pretty much disagree with Sanji's worldviews entirely here. Not that Niji isn't being a total asshat, but Sanji just goes to the stupid opposite extreme. Food shouldn't be frivolously wasted, but if you're eating a dish you dislike, you have every right to not finish it, even if it's technically well prepared. Just because Sanji's past experience has him conditioned to finish all his dishes as though it were a compulsion, that doesn't mean he has a right to force that compulsion on everyone regardless of the situation. And we all know my views on Sanji's regard toward women, so my disapproval there doesn't even need to be stated. Even the "you should never attack comrades ", something I normally agree with, is put into question here. A theory is going around that Germa 66 wants Big Mom to put souls into them. Well, if that means they do not currently have souls, then they're basically just fleshy robots. If that's the case, sure, why not, they're not real people, just automatons made of meat, I have no problem sacrificing them by the cart load.
That's kind of why I view portraying Niji as psychotically dickish as unnecessary. If he were just portrayed as a normal dude, he could still be Sanji's antithesis. But that's not enough for Oda, we also have to haaaaaaaate him, so he establishes his family's opposition to his idealogy in a such a horrible manner that no morally sane person could dare take his family's side. It definitely feels like such a depiction is the manga trying to force me to side with Sanji by default, because just look at them otherwise, when I feel neither character's idealogy is good.
Fortunately, I feel perfectly free to be morally insane within the confines of OP's fiction, so GO VINSMOKES, GO GO GO!
Started MHA. Up to chapter 12. So far, I think it's really good. Will post about it later in it's thread, but if anyone doesn't go there and is curious, I give it a thumbs up on it's introduction atleast.