i think Sanji fans doesn't hate Zoro, but they hate Zoro fans.
but with Zoro fans, they hate Sanji itself.
this happen because Sanji had more "flaws" than Zoro, but the constant hate from Zoro fans to Sanji make Sanji fans need to hate Zoro fans (not Zoro itself)
I think the hate stems from this:
Zoro fans tend to be battle junkies more often than other fans, so they love to overanalyze feats way too much and clearly care about feats a lot more than even Oda does. They hate the idea that Sanji is somewhat close to Zoro because they think Sanji is much weaker than he really is. They put too much emphasis on an injured Sanji getting his leg cracked a little by a Vet 400 chapters ago, Sanji getting “low diffed” by Doffy (mainly because Sanji didn’t know about DD’s abilities and got caught off guard), and according to some of them, Sanji looked like a weakling in WCI because he cried, and obviously strong willed people, who tend to be strong physically, don’t ever cry, so Sanji must not be strong willed and therefore not strong. They also erroneously think that Sanji’s weaker than he really is because he doesn’t have that “strong guy” stoic, badass personality. Rather than believe that Sanji being close to Zoro means Sanji’s just that strong, they interpret it as meaning Zoro is just that weak, which is why they vehemently deny Sanji being able to push Zoro to high diff despite the story portraying this as being the case