The only reason to doubt ZKK is Luffy. Oda for the last 20 years has always given Luffy the spotlight against the main villain. Moreso this post-timeskip than pre-skip. Luffy robbed Law of his final blow against Doffy(y'all can't that Luffy deserved to have defeated Doffy when Law's relationship against Doffy was far stronger than his) and he has made Luffy fight Kata just because. If there's a reason for why Zoro won't kill Kaido then this is it and that's fair to believe.
However, this arc has been very peculiar so far at least in regards to the main fights. It isn't just Luffy who has developed a hatred for Kaido. Now Zoro has too. Throughout Wano, we've seen Zoro slowly but surely develop his resentment towards Kaido. Seeing how the people of Wano have suffered, learning about Oden, and Yasu's death. Now, during the raid, Oda has continuously displayed Zoro's desire to reach Kaido in order to fight him. And now in the chapter, Zoro is now declaring to slay Kaido. This is huge because when have we ever see Zoro declare to defeat the main villain? Unless my memory is bonkers but I'm pretty certain he didn't do it for Arlong, Crocodile, Enel, Lucci, Moriah, Hody, Caesar, and Doffy. That type of stuff is reserved for Luffy, but now Zoro is saying it too.
I find it hard to believe that Oda is only doing this for Zoro to just reopen Kaido's scar. That much was obvious the moment Zoro got Enma. Not even Kid has gotten all this dedicated to him building hatred against Kaido.
There are simply just more points suggesting why Zoro will be the one to take off that man's head.