This is obviously based on my personal observations and analysis but aside from just very valid taste differences that make someone like or dislike a character I think it has to do with a much broader issue in how media is consumed nowadays. There are two streams of thinking that I believe intersect a lot and have resulted in this odd treatment of “moral complexity” in media. One is anti-intellectualism (I’m using this very loosely in its recent twitter meaning) where everything is not that deep and is bare bones and the other is sanitization and censorship (I know they yap a lot and it seems contradictory but a lot of the extensive explanations can be reduced to this is morally wrong irl or this repulses me thus it shouldn’t be portrayed in media). If you consider both either separately or together it becomes easier to see why people can’t seem to deal with moral complexity in media; if thing bad then how can thing be good and how can it be enjoyed? Sukuna and Kenjaku are written to be very directly “bad” so there is no shame in enjoying them because at the end of the day everyone knows this is “bad” (because we are all in kindergarten). Another thing which is pure speculation on my part is perhaps a general lack of strong moral character irl or let’s put another way, a helplessness to achieve results based on your moral values (which can be very easily explained by the state of politics and economy idk idk bla bla) but then that helplessness can be elevated on the internet by creating a false claim to moral superiority regarding what is and isn’t right in a piece of media and by admonishing that which you think is wrong now suddenly you are achieving a sense of fulfillment and results. “Yes!! I told people that Geto sucks definitively because he is like Hitler, feels good to be right man!” With no regard to the nature of fiction, the audacity of that comparison, and the complete fucking futility of it in the grand scheme of actually interacting with a work. Well that’s what I think at least holy yap.