Why would I be salty if I already said Zoro was going to fight Kaido before?
Classic Zoro fanboy lies.
Anyways, Killer is being there doesn't mean he is better than Sanji. Sanji also fought main villain of an arc; Doflamingo. Because it was necessary that only SH Pirate who could fight on the sky was Sanji not Zoro. It was even harder for Sanji to fight at the sky, because that was where Doflamingo had all the advantages, still E.Oda put Sanji in front of Doflamingo, and still forced him to use Parasite, which he also used against Jozu before on the land.
Since Zoro got Enma, someone needs to use that sword, which is why Zoro is there. And Zoro is there, E.Oda can't make Zoro as if he is in captain position, he brought another underling like Zoro, who was Killer. So that's the deal here.
The only important pirates are Luffy, Law and Kidd. Zoro and Killer are only there because Luffy and Kidd decided to fight Yonko.